doc: make mkdtemp example work on Windows

PR-URL: https://github.com/nodejs/node/pull/15408
Fixes: https://github.com/nodejs/node/issues/14960
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/fs.md

@@ -1133,10 +1133,10 @@ object with an `encoding` property specifying the character encoding to use.
 Example:
 
 ```js
-fs.mkdtemp('/tmp/foo-', (err, folder) => {
+fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => {
   if (err) throw err;
   console.log(folder);
-  // Prints: /tmp/foo-itXde2
+  // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
 });
 ```
 
@@ -1148,7 +1148,7 @@ the `prefix` *must* end with a trailing platform-specific path separator
 
 ```js
 // The parent directory for the new temporary directory
-const tmpDir = '/tmp';
+const tmpDir = os.tmpdir();
 
 // This method is *INCORRECT*:
 fs.mkdtemp(tmpDir, (err, folder) => {