docs: Added missing 'form' argument to class-based-views.txt. Thanks, gabrielgrant

    :attr:`~SingleObjectMixin.queryset` attributes, describing the type of

    object that the ModelForm is manipulating. The view also provides

    ``self.object``, the instance being manipulated. If the instance is being

    created, ``self.object`` will be ``None``

    created, ``self.object`` will be ``None``.

    **Mixins**

        validated. Returns :attr:`FormMixin.success_url` if it is provided;

        otherwise, attempts to use the ``get_absolute_url()`` of the object.

    .. method:: form_valid()

    .. method:: form_valid(form)

        Saves the form instance, sets the current object for the view, and

        redirects to :meth:`.get_success_url`.