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: #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>

Author: addaleax

doc/onboarding-extras.md

@@ -22,12 +22,12 @@
 | `lib/timers` | @fishrock123, @misterdjules |
 | `lib/util` | @bnoordhuis, @cjihrig, @evanlucas |
 | `lib/zlib` | @addaleax, @bnoordhuis, @indutny |
-| `src/async-wrap.*` | @trevnorris |
+| `src/async-wrap.*` | @nodejs/async_hooks |
 | `src/node_api.*` | @nodejs/n-api |
 | `src/node_crypto.*` | @nodejs/crypto |
 | `test/*` | @nodejs/testing |
 | `tools/eslint`, `.eslintrc` | @not-an-aardvark, @silverwind, @trott |
-| async_hooks | @nodejs/diagnostics |
+| async_hooks | @nodejs/async_hooks for bugs/reviews (+ @nodejs/diagnostics for API) |
 | performance | @nodejs/performance |
 | upgrading V8 | @nodejs/v8, @nodejs/post-mortem |
 | upgrading npm | @fishrock123, @MylesBorins |