Fix undefined variable in Quick overview example

NameError: name 'channel' is not defined

README.rst

@@ -130,7 +130,7 @@ Quick overview
         # Declare the video queue so that the messages can be delivered.
         # It is a best practice in Kombu to have both publishers and
         # consumers declare the queue.
-        video_queue(channel).declare()
+        video_queue(conn.channel()).declare()
 
         # produce
         with conn.Producer(exchange=media_exchange,