doc: add esm example for os

PR-URL: https://github.com/nodejs/node/pull/53604
Reviewed-By: Yagiz Nizipli <yagiz.nizipli@sentry.io>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/os.md

@@ -9,7 +9,11 @@
 The `node:os` module provides operating system-related utility methods and
 properties. It can be accessed using:
 
-```js
+```mjs
+import os from 'node:os';
+```
+
+```cjs
 const os = require('node:os');
 ```