Use <kbd> instead of <span class="keystroke">

This has some reasonably nice built-in presentation in modern browsers,
and may also yield a small accessibility improvement.

Fixes #3929

src/ch02-00-guessing-game-tutorial.md

@@ -682,13 +682,12 @@ in the expression refers to the original `guess` variable that contained the
 input as a string. The `trim` method on a `String` instance will eliminate any
 whitespace at the beginning and end, which we must do to be able to compare the
 string to the `u32`, which can only contain numerical data. The user must press
-<span class="keystroke">enter</span> to satisfy `read_line` and input their
-guess, which adds a newline character to the string. For example, if the user
-types <span class="keystroke">5</span> and presses <span
-class="keystroke">enter</span>, `guess` looks like this: `5\n`. The `\n`
-represents “newline.” (On Windows, pressing <span
-class="keystroke">enter</span> results in a carriage return and a newline,
-`\r\n`.) The `trim` method eliminates `\n` or `\r\n`, resulting in just `5`.
+<kbd>enter</kbd> to satisfy `read_line` and input their guess, which adds a
+newline character to the string. For example, if the user types <kbd>5</kbd> and
+presses <kbd>enter</kbd>, `guess` looks like this: `5\n`. The `\n` represents
+“newline.” (On Windows, pressing <kbd>enter</kbd> results in a carriage return
+and a newline, `\r\n`.) The `trim` method eliminates `\n` or `\r\n`, resulting
+in just `5`.
 
 The [`parse` method on strings][parse]<!-- ignore --> converts a string to
 another type. Here, we use it to convert from a string to a number. We need to
@@ -762,11 +761,11 @@ and run the program again. The program will now ask for another guess forever,
 which actually introduces a new problem. It doesn’t seem like the user can quit!
 
 The user could always interrupt the program by using the keyboard shortcut
-<span class="keystroke">ctrl-c</span>. But there’s another way to escape this
-insatiable monster, as mentioned in the `parse` discussion in [“Comparing the
-Guess to the Secret Number”](#comparing-the-guess-to-the-secret-number)<!--
-ignore -->: if the user enters a non-number answer, the program will crash. We
-can take advantage of that to allow the user to quit, as shown here:
+<kbd>ctrl</kbd>-<kbd>c</kbd>. But there’s another way to escape this insatiable
+monster, as mentioned in the `parse` discussion in [“Comparing the Guess to the
+Secret Number”](#comparing-the-guess-to-the-secret-number)<!-- ignore -->: if
+the user enters a non-number answer, the program will crash. We can take
+advantage of that to allow the user to quit, as shown here:
 
 <!-- manual-regeneration
 cd listings/ch02-guessing-game-tutorial/no-listing-04-looping/

src/ch03-05-control-flow.md

@@ -191,9 +191,9 @@ like this:
 ```
 
 When we run this program, we’ll see `again!` printed over and over continuously
-until we stop the program manually. Most terminals support the keyboard
-shortcut <span class="keystroke">ctrl-c</span> to interrupt a program that is
-stuck in a continual loop. Give it a try:
+until we stop the program manually. Most terminals support the keyboard shortcut
+<kbd>ctrl</kbd>-<kdb>c</kbd> to interrupt a program that is stuck in a continual
+loop. Give it a try:
 
 <!-- manual-regeneration
 cd listings/ch03-common-programming-concepts/no-listing-32-loop
@@ -213,10 +213,9 @@ again!
 ^Cagain!
 ```
 
-The symbol `^C` represents where you pressed <span
-class="keystroke">ctrl-c</span>. You may or may not see the word `again!`
-printed after the `^C`, depending on where the code was in the loop when it
-received the interrupt signal.
+The symbol `^C` represents where you pressed <kbd>ctrl</kbd>-<kbd>c</kbd>. You
+may or may not see the word `again!` printed after the `^C`, depending on where
+the code was in the loop when it received the interrupt signal.
 
 Fortunately, Rust also provides a way to break out of a loop using code. You
 can place the `break` keyword within the loop to tell the program when to stop

src/ch20-01-single-threaded.md

@@ -113,10 +113,10 @@ part of the `drop` implementation. Browsers sometimes deal with closed
 connections by retrying, because the problem might be temporary. The important
 factor is that we’ve successfully gotten a handle to a TCP connection!
 
-Remember to stop the program by pressing <span class="keystroke">ctrl-c</span>
-when you’re done running a particular version of the code. Then restart the
-program by invoking the `cargo run` command after you’ve made each set of code
-changes to make sure you’re running the newest code.
+Remember to stop the program by pressing <kbd>ctrl</kbd>-<kbd>c</kbd> when
+you’re done running a particular version of the code. Then restart the program
+by invoking the `cargo run` command after you’ve made each set of code changes
+to make sure you’re running the newest code.
 
 ### Reading the Request
 

src/ch20-03-graceful-shutdown-and-cleanup.md

@@ -3,10 +3,10 @@
 The code in Listing 20-20 is responding to requests asynchronously through the
 use of a thread pool, as we intended. We get some warnings about the `workers`,
 `id`, and `thread` fields that we’re not using in a direct way that reminds us
-we’re not cleaning up anything. When we use the less elegant <span
-class="keystroke">ctrl-c</span> method to halt the main thread, all other
-threads are stopped immediately as well, even if they’re in the middle of
-serving a request.
+we’re not cleaning up anything. When we use the less elegant
+<kbd>ctrl</kbd>-<kbd>c</kbd> method to halt the main thread, all other threads
+are stopped immediately as well, even if they’re in the middle of serving a
+request.
 
 Next, then, we’ll implement the `Drop` trait to call `join` on each of the
 threads in the pool so they can finish the requests they’re working on before