Make "helppage" link relative

Spotted by Wmat
https://www.mediawiki.org/wiki/Thread:Extension_talk:ConfirmAccount/BUG:_Link_to_mediawiki.org_help_pages_is_internal_but_should_be_external_%282%29

Bug: 65454
Change-Id: I7f905557acbb3ae985b596fd8107945c0fbf6463
(cherry picked from commit 6795ac21071cb3963dabc4c9db2209c1e35ddd4c)