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.

@nodejs/diagnostics fyi – I know it was originally requested that you are the team to ping for this kind of thing, but I think for implementation issues it’s just easier to ping the subset that actually works on them. Anybody who wants to join @nodejs/async_hooks is welcome to do so!