doc: add example code for fs.existsSync()

PR-URL: #28354
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: nicolasrestrepo

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