Fixed database update instructions

As per our discussion here: https://github.com/tomchristie/django-rest-framework/pull/2102/
Right, missed that point.
We should probably add a note that this isn't a good practice (something such as for the sake of simplicity we do it but don't do that at home). Tutorials are often referred to and I'm not happy with removing the database and migrations to recreate both.
I'm ok for the only reason that doing it the right way will introduce more complexity (default value for the fields).