Fix agent's link to standard comm channels list

Author: btalb

benchbot_api/agent.py

@@ -35,7 +35,7 @@ def pick_action(self, observations, action_list):
 
         The current list of possible actions (& required arguments) is
         available here:
-            https://github.com/qcr/benchbot_api#default-communication-channel-list
+            https://github.com/qcr/benchbot_api#standard-communication-channels
         """
         return None