[1.6.x] Fixed QuerySet datetimes argument name in docs.

.. versionadded:: 1.6

.. method:: datetimes(field, kind, order='ASC', tzinfo=None)

.. method:: datetimes(field_name, kind, order='ASC', tzinfo=None)

Returns a ``DateTimeQuerySet`` — a ``QuerySet`` that evaluates to a list of

:class:`datetime.datetime` objects representing all available dates of a

particular kind within the contents of the ``QuerySet``.

``field`` should be the name of a ``DateTimeField`` of your model.

``field_name`` should be the name of a ``DateTimeField`` of your model.

``kind`` should be either ``"year"``, ``"month"``, ``"day"``, ``"hour"``,

``"minute"`` or ``"second"``. Each ``datetime.datetime`` object in the result