Fix grammatical error in...

Fix grammatical error in ActiveModel::MassAssignmentSecurity::PermissionSet#deny? NotImplementedError message

activemodel/lib/active_model/mass_assignment_security/permission_set.rb

@@ -13,7 +13,7 @@ def include?(key)
       end
 
       def deny?(key)
-        raise NotImplementedError, "#deny?(key) suppose to be overwritten"
+        raise NotImplementedError, "#deny?(key) supposed to be overwritten"
       end
 
     protected