Make Custom Validators example more clear

The Active Record Validations guide use `Person < ApplicationRecord`
in all examples. When reading about Custom Validators, one of the
examples had a different configuration. By using the same example
everywhere, this change helps the user save time and feel more confident
using the feature.

Author: Stefanni Brasil

guides/source/active_record_validations.md

@@ -1078,8 +1078,7 @@ class MyValidator < ActiveModel::Validator
   end
 end
 
-class Person
-  include ActiveModel::Validations
+class Person < ApplicationRecord
   validates_with MyValidator
 end
 ```