doc: fix punctuation issue in async_hooks.md

Move period incorrectly placed outside of parentheses to inside the
parentheses. The parenthetical in this case is a full sentence.

PR-URL: https://github.com/nodejs/node/pull/19364


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>