datetime.rst: fix combine() signature (#101490)

The default `tzinfo` param of the `combine()` signature pseudocode was erroneously `self.tzinfo`.

`self` has no meaning in the context of a classmethod, and the datetime class itself has no `tzinfo` attribute.  The correct default pseudocode is `time.tzinfo`, reflecting that the default is the `tzinfo` attribute of the `time` parameter.