Fix ActiveModel::Serializers::Xml#to_xml reference

Link to ActiveRecord::Serialization#to_xml for more details

activemodel/lib/active_model/serializers/xml.rb

@@ -182,7 +182,7 @@ def add_procs
       #
       # To include associations use <tt>:include</tt>.
       #
-      # For further documentation, see activerecord/lib/active_record/serializers/xml_serializer.xml.
+      # For further documentation, see ActiveRecord::Serialization#to_xml
       def to_xml(options = {}, &block)
         Serializer.new(self, options).serialize(&block)
       end