Doc issue: specifying redis backend

[mclarknc]

I love this project! It solves a couple of big problems I've been having.

This is a nit, but when specifying the redis backend, the current docs say:
CHANNEL_BACKENDS = {
"default": {
"BACKEND": "channels.backends.redis.RedisChannelBackend",

I had to use:
"BACKEND": "channels.backends.redis_py.RedisChannelBackend",