doc: fix info string causing duplicated code blocks

Two fenced code blocks were incorrectly labeled as `js` instead of
`cjs`, causing both ESM and CJS version to be shown on
https://nodejs.org/api/test.html#timers instead of being conditionally
shown depending on the value of the "CJS / ESM" toggle.

PR-URL: nodejs#52660
Reviewed-By: Xuguang Mei <meixuguang@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: mleenhardt

doc/api/test.md

@@ -641,7 +641,7 @@ test('mocks setTimeout to be executed synchronously without having to actually w
 });
 ```
 
-```js
+```cjs
 const assert = require('node:assert');
 const { mock, test } = require('node:test');
 
@@ -660,7 +660,7 @@ test('mocks setTimeout to be executed synchronously without having to actually w
   // Reset the globally tracked mocks.
   mock.timers.reset();
 
-  // If you call reset mock instance, it'll also reset timers instance
+  // If you call reset mock instance, it will also reset timers instance
   mock.reset();
 });
 ```
@@ -688,7 +688,7 @@ test('mocks setTimeout to be executed synchronously without having to actually w
 });
 ```
 
-```js
+```cjs
 const assert = require('node:assert');
 const { test } = require('node:test');