doc: update link in onboarding doc

PR-URL: https://github.com/nodejs/node/pull/40539


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>