doc nit: slug_field not required

[CarlFK]

​http://docs.djangoproject.com/en/dev/ref/generic-views/#django-views-generic-list-detail-object-detail

seems "and slug_field" is not required given " By default, slug_field is 'slug'."

"""
Either object_id or (slug and slug_field) is required.

If you provide object_id, it should be the value of the primary-key field for the object being displayed on this page.

Otherwise, slug should be the slug of the given object, and slug_field should be the name of the slug field in the QuerySet's model. By default, slug_field is 'slug'.
"""

[Tim Graham]

The current wording makes sense to me -- it shows that slug and slug_field are used together if object_id isn't specified. Thanks for the suggestion though.