Fixed typo in docs/topics/db/models.txt.

As mentioned, Django will automatically create a

:class:`~django.db.models.OneToOneField` linking your child

class back any non-abstract parent models. If you want to control the

class back to any non-abstract parent models. If you want to control the

name of the attribute linking back to the parent, you can create your

own :class:`~django.db.models.OneToOneField` and set

:attr:`parent_link=True <django.db.models.OneToOneField.parent_link>`