doc: update main branch name in onboarding.md

PR-URL: nodejs/node#43443
Refs: nodejs/node#33864
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

onboarding.md

@@ -41,7 +41,7 @@ onboarding session.
   * Add the canonical nodejs repository as `upstream` remote:
     * `git remote add upstream git@github.com:nodejs/node.git`
   * To update from `upstream`:
-    * `git checkout master`
+    * `git checkout main`
     * `git fetch upstream HEAD`
     * `git reset --hard FETCH_HEAD`
   * Make a new branch for each pull request you submit.