Fix associations presence lost in steve's reformat

https://github.com/lifo/docrails/commit/55a2820cc6d33e96b8d1b64b38b033913058dce4
messes up
https://github.com/lifo/docrails/commit/cfd324b4b68469ba3188e4b7ba8586e59b239693

Hopefully there is other stuff lost in translation...

guides/source/active_record_validations.md

@@ -503,8 +503,8 @@ 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.
+whether the associated object itself is present, and not the foreign key used
+to map the association.
 
 ```ruby
 class LineItem < ActiveRecord::Base