test,doc: adjust async-hooks coverage doc for lint

First header should be a first-level header according to our lint rules.
Make it so in prepartion for applying the markdown linting to the test
directory.

PR-URL: https://github.com/nodejs/node/pull/22221


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>