Make user group related messages consistent

Reported by Liuxinyu970226 at
https://translatewiki.net/w/i.php?title=Thread:Support/About_MediaWiki:Grouppage-flow-bot/en,_MediaWiki:Group-flow-bot-member/en_%26_MediaWiki:Group-flow-bot/en

Change-Id: I378fa10ed8e381197bff13d17bb81e98924376ef