Find group inside budget

We were having a problem were some groups where not updating correctly.

That was because it was finding the first group with that name. However
we were looking for another group with the same name from another budget

Apart from the group_id, we also get the budget_id in the params for
updating a group. By finding groups within that budget we get the
expected behaviour

app/controllers/admin/budget_groups_controller.rb

@@ -9,7 +9,7 @@ class Admin::BudgetGroupsController < Admin::BaseController
   end
 
   def update
-    @group = Budget::Group.by_slug(params[:id]).first
+    @group = @budget.groups.by_slug(params[:id]).first
     @group.update(budget_group_params)
   end