[1.8.x] Fixed minor typo in Cryptography section of 1.8 release notes

^^^^^^^^^^^^

* The ``max_age`` parameter of the

  :meth:`django.core.signing.TimestampSigner.unsign` method now also accept a

  :meth:`django.core.signing.TimestampSigner.unsign` method now also accepts a

  :py:class:`datetime.timedelta` object.

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