Update GroupAdmin.php

Without the text in the 'with' method the symfony was throwing:

ContextErrorException: Warning: Missing argument 1 for Sonata\AdminBundle\Mapper\BaseGroupedMapper::with(), called in /var/www/html/muva/vendor/sonata-project/user-bundle/Admin/Model/GroupAdmin.php on line 68 and defined in /var/www/html/muva/app/cache/dev/classes.php line 12165