Fix validation based on object not _id.

From https://github.com/rails/rails/issues/6161\#issuecomment-10750118

guides/source/active_record_validations_callbacks.md

@@ -372,12 +372,12 @@ class Person < ActiveRecord::Base
 end
 ```
 
-If you want to be sure that an association is present, you'll need to test whether the foreign key used to map the association is present, and not the associated object itself.
+If you want to be sure that an association is present, you'll need to test the associated object itself, and not whether the foreign key used to map the association is present:
 
 ```ruby
 class LineItem < ActiveRecord::Base
   belongs_to :order
-  validates :order_id, presence: true
+  validates :order, presence: true
 end
 ```