Fixed #24674 -- Documented the correct NullBooleanField formfield.

:class:`ManyToManyField`            :class:`~django.forms.ModelMultipleChoiceField`

                                    (see below)

:class:`NullBooleanField`           :class:`~django.forms.CharField`

:class:`NullBooleanField`           :class:`~django.forms.NullBooleanField`

:class:`PositiveIntegerField`       :class:`~django.forms.IntegerField`