doc: fix minor issues in async_hooks.md

* easily -> easy
* was -> is
* add a missing comma

PR-URL: https://github.com/nodejs/node/pull/19313


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>