Update dirty.rb: documentation fix.

ActiveModel::Dirty module documentation fix.

activemodel/lib/active_model/dirty.rb

@@ -26,6 +26,10 @@ module ActiveModel
   #
   #     define_attribute_methods :name
   #
+  #     def initialize(name)
+  #       @name = name
+  #     end
+  #
   #     def name
   #       @name
   #     end
@@ -54,7 +58,7 @@ module ActiveModel
   #
   # A newly instantiated +Person+ object is unchanged:
   #
-  #   person = Person.new
+  #   person = Person.new(name: "Uncle Bob")
   #   person.changed? # => false
   #
   # Change the name: