doc: update repository list in onboarding doc

LTS repo is now Release.

readable-stream is important but a bit of a specialized topic. The
others listed here are of general interest.

PR-URL: nodejs/node#44089
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Feng Yu <F3n67u@outlook.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

onboarding.md

@@ -241,8 +241,7 @@ needs to be pointed out separately during the onboarding.
   * <https://github.com/nodejs/TSC>
   * <https://github.com/nodejs/build>
   * <https://github.com/nodejs/nodejs.org>
-  * <https://github.com/nodejs/readable-stream>
-  * <https://github.com/nodejs/LTS>
+  * <https://github.com/nodejs/Release>
   * <https://github.com/nodejs/citgm>
 * The OpenJS Foundation hosts regular summits for active contributors to the
   Node.js project, where we have face-to-face discussions about our work on the