Fix a rather fatal typo in rebuildrecentchanges.php

Fix a rather fatal typo in rebuildrecentchanges.php

The JOIN condition was being ignored because there is no table called "user_group" in MediaWiki core.
Thus if and when using $wgSharedDB, the query would end up listing *all* registered users from the shared user table.
And even without $wgSharedDB, running rebuildrecentchanges.php would result in everyone's edits being marked as bot edits (recentchanges.rc_bot = 1) and thus hidden from the Special:RecentChanges page.

Thanks to Lcawte for reporting this bug.

Follow-up to 27c61fb1e94da9114314468fd00bcf129ec064b6

Change-Id: I18d658b67c50f2200341f732783c2e7524dd27f1