doc: update who to cc for async_hooks

Now that `async_hooks` is released, it makes sense to me to have a team
for addressing issues with it. nodejs/diagnostics can still be
@mentioned if there are questions about the API rather than its
implementation.

PR-URL: https://github.com/nodejs/node/pull/13332


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>