doc: fix test runner's only tests section header

Only tests are not a subsection of describe/it syntax.
This commit updates the heading of the 'only tests'
section to reflect this.

PR-URL: #45343
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>

doc/api/test.md

@@ -184,7 +184,7 @@ import { describe, it } from 'node:test';
 const { describe, it } = require('node:test');
 ```
 
-### `only` tests
+## `only` tests
 
 If Node.js is started with the [`--test-only`][] command-line option, it is
 possible to skip all top level tests except for a selected subset by passing