Fixed broken, memoized attributes method example

[#6245 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

activemodel/lib/active_model/serialization.rb

@@ -15,7 +15,7 @@ module ActiveModel
   #     attr_accessor :name
   #
   #     def attributes
-  #       @attributes ||= {'name' => nil}
+  #       {'name' => name}
   #     end
   #
   #   end
@@ -45,7 +45,7 @@ module ActiveModel
   #     attr_accessor :name
   #
   #     def attributes
-  #       @attributes ||= {'name' => nil}
+  #       {'name' => name}
   #     end
   #
   #   end