bp-legacy: For member groups template, use same `<select>` ID as the groups directory template.

This is done for consistency from a theme developer's perspective.

Props lenasterg.  Fixes #5998.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9232 cdf35c40-ae34-48e0-9cc9-0c9da1808c22