Fixed #20654 -- Fixed type of `m2m_changed`'s `pk_set` arg in docs

``pk_set``

    For the ``pre_add``, ``post_add``, ``pre_remove`` and ``post_remove``

    actions, this is a list of primary key values that have been added to

    actions, this is a set of primary key values that have been added to

    or removed from the relation.

    For the ``pre_clear`` and ``post_clear`` actions, this is ``None``.

``model``       ``Topping`` (the class of the objects added to the

                ``Pizza``)

``pk_set``      ``[t.id]`` (since only ``Topping t`` was added to the relation)

``pk_set``      ``set([t.id])`` (since only ``Topping t`` was added to the relation)

``using``       ``"default"`` (since the default router sends writes here)

==============  ============================================================

``model``       ``Pizza`` (the class of the objects removed from the

                ``Topping``)

``pk_set``      ``[p.id]`` (since only ``Pizza p`` was removed from the

``pk_set``      ``set([p.id])`` (since only ``Pizza p`` was removed from the

                relation)

``using``       ``"default"`` (since the default router sends writes here)