fixed 'Objectset' typo in avocado.sets.models

I found this when I tried to test the latest Avocado with the pcgc-varify integration.  However, even when fixed, note that Varify thinks that ObjectSet has a `name` field, which it does not in django-objectset, so this compatibility shim doesn't help with Varify at the moment.  I'm not sure where this latter issue should be raised.