doc: use Class: consistently

The docs contain 122 uses of `Class:` in headers and one use of `class:`
in headers. This changes that one instance to conform with the other
instances.

PR-URL: #33978
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/fs.md

@@ -4328,7 +4328,7 @@ The `fs.promises` API provides an alternative set of asynchronous file system
 methods that return `Promise` objects rather than using callbacks. The
 API is accessible via `require('fs').promises`.
 
-### class: `FileHandle`
+### Class: `FileHandle`
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