Name the argument according with its job

Related with dc1b937d

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -334,8 +334,8 @@ def path_ast(path)
             parser.parse path
           end
 
-          def dispatcher(defaults)
-            @set.dispatcher defaults
+          def dispatcher(raise_on_name_error)
+            @set.dispatcher raise_on_name_error
           end
       end