Fixed: DebugGroup and DebugUser not working

When in debug mode you have links to the debuggroup and debuguser is the com_user views.
These give 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/users view (administrator/index.php?option=com_users&view=debuggroup & administrator/index.php?option=com_users&view=debuguser)