doc: use "repository" instead of "repo" in onboarding.md

PR-URL: #39286
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

onboarding.md

@@ -53,8 +53,8 @@ onboarding session.
 * Notifications:
   * Use [https://github.com/notifications](https://github.com/notifications) or
     set up email
-  * Watching the main repo will flood your inbox (several hundred notifications
-    on typical weekdays), so be prepared
+  * Watching the main repository will flood your inbox (several hundred
+    notifications on typical weekdays), so be prepared
 
 The project has two venues for real-time discussion:
 * [`#nodejs-dev`](https://openjs-foundation.slack.com/archives/C019Y2T6STH) on
@@ -179,8 +179,8 @@ The project has two venues for real-time discussion:
       `7006` in the `PR_ID`.
     * The remaining elements on the form are typically unchanged.
   * If you need help with something CI-related:
-    * Use the [Build WG repo](https://github.com/nodejs/build) to file issues
-      for the Build WG members who maintain the CI infrastructure.
+    * Use the [Build WG repository](https://github.com/nodejs/build) to file
+      issues for the Build WG members who maintain the CI infrastructure.
 
 ## Landing PRs