Fix i18n issue in Special:NewImages

As Rillke has reported in
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Showhidebots/en
the text for the check box was incorrect. It turned out to be a strange
re-use of messages. Replaced it with a proper message.

No re-uses in core. Remaining re-use outside of core has been taken care of
in I4458ab7f6e53

Change-Id: I48042d0e60ce213db3db123179d9658cdd013a8f