doc: use _Class Method_ in async_hooks.md

We use _Class Method_ rather than _static_ in documentation headers in
buffer.md and crypto.md.  We use _static_ in one place in
async_hooks.md. Change to _Class Method_ for consistency.

PR-URL: https://github.com/nodejs/node/pull/34626


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>