[1.8.x] Fixed typo in current_app deprecation note.

      .. deprecated:: 1.8

          The ``curent_app`` argument is deprecated. If you need it, you must

          The ``current_app`` argument is deprecated. If you need it, you must

          now use a :class:`RequestContext` instead of a :class:`Context`.

    For details, see :ref:`playing-with-context` below.