Fixed: DebugGroupnot working

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31229

When in debug mode you have links to the debuggroup is the com_user views.
This gives an error: "500 Layout default not found".
This is caused by the capital in the classname (UsersViewDebugGroup vs UsersViewDebuggroup).

This PR fixes that issue.

#### Testing Instructions
Switch on debugging in the configuration.
Go to the dubug groups view (administrator/index.php?option=com_users&view=debuggroup