it's xml, not json

change docs for xml serializer to talk about xml

activemodel/lib/active_model/serializers/xml.rb

@@ -205,7 +205,7 @@ module ActiveModel
         Serializer.new(self, options).serialize(&block)
       end
 
-      # Sets the model +attributes+ from a JSON string. Returns +self+.
+      # Sets the model +attributes+ from an XML string. Returns +self+.
       #
       #   class Person
       #     include ActiveModel::Serializers::Xml