Fix example code in `doc/general/getting_started.md`
Fix example code in `doc/general/getting_started.md`

The `belongs_to` method should take relation name, not a foreign_key property.

ScottKbka/active_model_serializers,nurey/active_model_serializers,ScottKbka/active_model_serializers,nurey/active_model_serializers,cassidycodes/active_model_serializers,nurey/active_model_serializers,ScottKbka/active_model_serializers,rails-api/active_model_serializers,fabianoleittes/active_model_serializers