Update examples in scoped.rb to indicated that FriendlyId::Scoped has to be included before it'll work

Hey guys, 

Not sure this is right, but I only got scopes working by having "include FriendlyId::Scoped" in my model. 

Otherwise I'd get an error like: friendly_id-4.0.9/lib/friendly_id/configuration.rb:77:in `block in set': undefined method `scope='

Submitting a pull request to start the discussion, if its wrong, no worries.