Improve AMo changelog example to not rely on AR [ci skip]

activemodel/CHANGELOG.md

@@ -2,17 +2,21 @@
 *   Add `ActiveModel::Validations::AbsenceValidator`, a validator to check the
     absence of attributes.
 
-        class Person < ActiveRecord::Base
+        class Person
+          include ActiveModel::Validations
+
+          attr_accessor :first_name
           validates_absence_of :first_name
         end
 
         person = Person.new
         person.first_name = "John"
         person.valid?
-        => false
-        # first_name must be blank
+        # => false
+        person.errors.messages
+        # => {:first_name=>["must be blank"]}
 
-    * Roberto Vasquez Angel*
+    *Roberto Vasquez Angel*
 
 *   Added `ActiveModel::Errors#add_on_present` method. Adds error messages to
     present attributes.