doc: fix mkdtemp() documentation

Several minor fixes to the entries for `mkdtemp()`. The most significant
is that a mistaken use of `fs.mkdtemp()` is corrected to
`fsPromises.mkdtemp()`.

PR-URL: #20512
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>

doc/api/fs.md

@@ -2064,8 +2064,6 @@ parameter.
 The optional `options` argument can be a string specifying an encoding, or an
 object with an `encoding` property specifying the character encoding to use.
 
-Example:
-
 ```js
 fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => {
   if (err) throw err;
@@ -2077,7 +2075,7 @@ fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => {
 The `fs.mkdtemp()` method will append the six randomly selected characters
 directly to the `prefix` string. For instance, given a directory `/tmp`, if the
 intention is to create a temporary directory *within* `/tmp`, the `prefix`
-*must* end with a trailing platform-specific path separator
+must end with a trailing platform-specific path separator
 (`require('path').sep`).
 
 ```js
@@ -4003,24 +4001,22 @@ added: v10.0.0
   * `encoding` {string} **Default:** `'utf8'`
 * Returns: {Promise}
 
-Creates a unique temporary directory then resolves the `Promise` with the
-created folder path. A unique directory name is generated by appending six
-random characters to the end of the provided `prefix`.
+Creates a unique temporary directory and resolves the `Promise` with the created
+folder path. A unique directory name is generated by appending six random
+characters to the end of the provided `prefix`.
 
 The optional `options` argument can be a string specifying an encoding, or an
 object with an `encoding` property specifying the character encoding to use.
 
-Example:
-
 ```js
 fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
   .catch(console.error);
 ```
 
-The `fs.mkdtemp()` method will append the six randomly selected characters
-directly to the `prefix` string. For instance, given a directory `/tmp`, if the
-intention is to create a temporary directory *within* `/tmp`, the `prefix`
-*must* end with a trailing platform-specific path separator
+The `fsPromises.mkdtemp()` method will append the six randomly selected
+characters directly to the `prefix` string. For instance, given a directory
+`/tmp`, if the intention is to create a temporary directory *within* `/tmp`, the
+`prefix` must end with a trailing platform-specific path separator
 (`require('path').sep`).
 
 ### fsPromises.open(path, flags[, mode])