Fix private call (#446)

This should make method `relationship_option` private instead `relationship_option`.

Co-authored-by: Jeff Keen <[email protected]>

lib/graphiti/resource/dsl.rb

@@ -205,7 +205,7 @@ def relationship_option(options, name)
             options[name] ||= send(:"relationships_#{name}_by_default")
           end
         end
-        private :attribute_option
+        private :relationship_option
       end
     end
   end