Slack bridge: Clarify config placeholder content.

Author: rht

zulip/integrations/bridge_with_slack/bridge_with_slack_config.py

@@ -7,8 +7,8 @@
         "topic": "<- slack-bridge",
     },
     "slack": {
-        "username": "slack username",
-        "token": "slack token",
+        "username": "slack_username",
+        "token": "xoxb-your-slack-token",
         "channel": "C5Z5N7R8A -- must be channel id",
     },
 }