doc: fix nits in code examples of async_hooks.md

* Make `require()` consistent.
* Add missing argument.
* Add missing `\n` in outputs.
* Reduce string concatenations.
* Update outputs.
* Reword and fix a typo.

PR-URL: https://github.com/nodejs/node/pull/13400


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>