doc: update main branch name in onboarding.md

PR-URL: #43443
Refs: #33864
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: LiviaMedeiros <[email protected]>
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: F3n67u

onboarding.md

@@ -41,7 +41,7 @@ onboarding session.
   * Add the canonical nodejs repository as `upstream` remote:
     * `git remote add upstream [email protected]: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.