compose: fix whiteboard collab server variable name

Fixes: https://github.com/jitsi/docker-jitsi-meet/issues/1469

docker-compose.yml

@@ -150,7 +150,7 @@ services:
             - XMPP_RECORDER_DOMAIN
             - XMPP_PORT
             - WHITEBOARD_ENABLED
-            - WHITEBOARD_COLLAB_SERVER_BASE_URL
+            - WHITEBOARD_COLLAB_SERVER_PUBLIC_URL
         networks:
             meet.jitsi: