doc: fix lint errors in packages.md

Code samples that use CJS-only syntax need to use the cjs markdown
identifiers.

PR-URL: #39792
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>

Author: Trott

doc/api/packages.md

@@ -720,12 +720,12 @@ code will also work:
 }
 ```
 
-```js
+```cjs
 // ./index.js
 module.exports = 42;
 ```
 
-```js
+```cjs
 // ./other.js
 console.log(require('@my/package'));
 ```