Fixed typo in docs. Thanks Berker.

  will be removed in Django 2.1. The more general ``check_expression_support()``

  should be used instead.

``django.forms.extras`` is deprecated. You can find

* ``django.forms.extras`` is deprecated. You can find

  :class:`~django.forms.SelectDateWidget` in ``django.forms.widgets``

  (or simply ``django.forms``) instead.