respond_to -> respond to in a message from AM::Lint

The error message uses English to say that the object should respond
to a certain method. You check that with respond_to, but that's not
English.

This makes this error message also consistent with the rest.

activemodel/lib/active_model/lint.rb

@@ -98,7 +98,7 @@ def test_errors_aref
 
       private
         def model
-          assert @model.respond_to?(:to_model), "The object should respond_to to_model"
+          assert @model.respond_to?(:to_model), "The object should respond to to_model"
           @model.to_model
         end