Docs: Update encode/decode methods in example.
Docs: Update encode/decode methods in example.

`encode` is now `channel_encode`, and `decode` is now `channel_decode`.

linuxlewis/channels,andrewgodwin/django-channels,Krukov/channels,raphael-boucher/channels,Coread/channels,andrewgodwin/channels,django/channels,Coread/channels,raiderrobert/channels,Krukov/channels