doc: apply sentence case to headers in doc/guides

PR-URL: #37478
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/technical-values.md

@@ -65,7 +65,7 @@ Some key elements of this include:
 * Low-friction policies and processes
 * Good CI and tooling to make maintainers productive
 
-### 5 - Up to date Technology and APIs
+### 5 - Up to date technology and APIs
 We value providing developers with modern APIs and technologies
 following existing standards whenever possible.
 Some key elements of this include:

doc/guides/using-internal-errors.md

@@ -1,4 +1,4 @@
-# Using the internal/errors.js Module
+# Using the internal/errors.js module
 
 ## What is internal/errors.js
 

doc/guides/writing-tests.md

@@ -229,7 +229,7 @@ argument. It is not a good idea to simply pass `common.mustCall()` to those
 because `common.mustCall()` will ignore the error. Use `common.mustSucceed()`
 instead.
 
-#### Countdown Module
+#### Countdown module
 
 The common [Countdown module](https://github.com/nodejs/node/tree/HEAD/test/common#countdown-module)
 provides a simple countdown mechanism for tests that require a particular
@@ -345,7 +345,7 @@ in each release, such as:
 * Template literals over string concatenation
 * Arrow functions when appropriate
 
-## Naming Test Files
+## Naming test files
 
 Test files are named using kebab casing. The first component of the name is
 `test`. The second is the module or subsystem being tested. The third is usually
@@ -357,13 +357,13 @@ named `test-process-before-exit.js`. If the test specifically checked that arrow
 functions worked correctly with the `beforeExit` event, then it might be named
 `test-process-before-exit-arrow-functions.js`.
 
-## Imported Tests
+## Imported tests
 
-### Web Platform Tests
+### Web platform tests
 
 See [`test/wpt`](../../test/wpt/README.md) for more information.
 
-## C++ Unit test
+## C++ unit test
 
 C++ code can be tested using [Google Test][]. Most features in Node.js can be
 tested using the methods described previously in this document. But there are
@@ -444,7 +444,7 @@ and tearing it down after the tests have finished.
 It also contains a helper to create arguments to be passed into Node.js. It
 will depend on what is being tested if this is required or not.
 
-### Test Coverage
+### Test coverage
 
 To generate a test coverage report, see the
 [Test Coverage section of the Building guide][].