Fix typo in property name

Change I270bde41 introduced a typo to RecentChanges.php, referring to
'$this->mExtras' when '$this->mExtra' was intended. This caused the IRC
log action comment to be omitted from the IRC RC feed.

Bug: 53720
Change-Id: I7f29ba8392861e00a37a2196833154bac41298b4