Fixed typo in JSONResponseMixin example.

For example, a simple JSON mixin might look something like this::

    import json

    from django import http

    from django.http import HttpResponse

    class JSONResponseMixin(object):

        """

        A mixin that can be used to render a JSON response.

        """

        reponse_class = HTTPResponse

        response_class = HttpResponse

        def render_to_response(self, context, **response_kwargs):

            """