[1.6.x] Fixed typo in docs/ref/request-response.txt.

   Instantiates a ``QueryDict`` object based on ``query_string``.

          >>> QueryDict('a=1&a=2&c=3')

          <QueryDict: {u'a': [u'1', u'2'], u'b': [u'1']}>

          <QueryDict: {'a': ['1', '2'], 'c': ['3']}>

   Most ``QueryDict``\ s you encounter, and in particular those at

   ``request.POST`` and ``request.GET``, will be immutable. If you are