doc: fix link in onboarding.md

Remove link to the outdated members team

PR-URL: https://github.com/nodejs/node/pull/18878


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>