Update pot file after 'group_activity' new member sort order introduction

The new sort order for group member queries generated a new string 'Group Activity'. As a result using 'grunt build-commit' generates a new buddypress.pot file.

see #3430'


git-svn-id: https://buddypress.svn.wordpress.org/trunk@8398 cdf35c40-ae34-48e0-9cc9-0c9da1808c22