doc: update fs.md

First class heading is made to look like other headings.

PR-URL: #33820
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/api/fs.md

@@ -284,7 +284,7 @@ synchronous use libuv's threadpool, which can have surprising and negative
 performance implications for some applications. See the
 [`UV_THREADPOOL_SIZE`][] documentation for more information.
 
-## Class `fs.Dir`
+## Class: `fs.Dir`
 <!-- YAML
 added: v12.12.0
 -->