Corrects incorrect param name in Groups Admin autocomplete AJAX handler

'group_type' was a typo for 'group_role'. As a result, the autocomplete was
excluding Members only, rather than Members + Mods + Admins.

Fixes #5076

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7242 cdf35c40-ae34-48e0-9cc9-0c9da1808c22