doc: require `node:process` in assert doc examples

PR-URL: https://github.com/nodejs/node/pull/53702
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>

doc/api/assert.md

@@ -269,6 +269,7 @@ process.on('exit', () => {
 
 ```cjs
 const assert = require('node:assert');
+const process = require('node:process');
 
 const tracker = new assert.CallTracker();