doc: add example code for fs.existsSync()

PR-URL: https://github.com/nodejs/node/pull/28354
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -1763,6 +1763,11 @@ this API: [`fs.exists()`][].
 parameter to `fs.exists()` accepts parameters that are inconsistent with other
 Node.js callbacks. `fs.existsSync()` does not use a callback.
 
+```js
+if (fs.existsSync('/etc/passwd')) {
+  console.log('The file exists.');
+}
+```
 
 ## fs.fchmod(fd, mode, callback)
 <!-- YAML