doc: fix author-ready conflict

The onboarding docs still contained an outdated description of the
`author-ready` label. Now it references the main description to
prevent any future conflicts.

PR-URL: https://github.com/nodejs/node/pull/25015


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>