doc: correct styling of _GitHub_ in onboarding doc

PR-URL: #21361
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/onboarding.md

@@ -144,7 +144,7 @@ onboarding session.
     (especially if it just has nits left).
 * Approving a change
   * Collaborators indicate that they have reviewed and approve of the changes in
-    a pull request using Github’s approval interface
+    a pull request using GitHub’s approval interface
   * Some people like to comment `LGTM` (“Looks Good To Me”)
   * You have the authority to approve any other collaborator’s work.
   * You cannot approve your own pull requests.