Update example config

Co-authored-by: Tulir Asokan <tulir@maunium.net>

mautrix_signal/example-config.yaml

@@ -206,7 +206,7 @@ bridge:
         # Sent when joining a management room and the user is not logged in.
         welcome_unconnected: "Use `help` for help or `register` to log in."
         # Optional extra text sent when joining a management room.
-        # additional_help: "This would be some additional text in case you need it."
+        additional_help: ""
 
     # Send each message separately (for readability in some clients)
     management_room_multiple_messages: false