Normalize theme name “Respondr” in PAGS group name

Name the PAGS group of users of the Respondr theme “Respondr Theme Users” rather than “Responder Theme Users”.

This bit me in testing in that I searched Groups for "Respondr", found nothing, and wrongly concluded that something was wrong with the loading of the group representing users of the Respondr theme, whereas all that was really at work was a difference in spelling of "Respondr".  I believe "Respondr" is the preferred spelling, standardizing on that.