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: https://github.com/nodejs/node/pull/33978


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