doc: add esm example for os

Author: Leonardo Peixoto

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');
 ```