doc: add example for chmod in fs.md

PR-URL: https://github.com/nodejs/node/pull/28365
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/fs.md

@@ -1243,6 +1243,13 @@ possible exception are given to the completion callback.
 
 See also: chmod(2).
 
+```js
+fs.chmod('my_file.txt', 0o775, (err) => {
+  if (err) throw err;
+  console.log('The permissions for file "my_file.txt" have been changed!');
+});
+```
+
 ### File modes
 
 The `mode` argument used in both the `fs.chmod()` and `fs.chmodSync()`