fix(socket): variable name wasn't the same

TODO add test on them web sockets

backend/matcha/websocket/socket_manager.py

@@ -29,7 +29,7 @@ def add_session(cls, sid, user_id):
     @classmethod
     def remove_session(cls, sid):
         user_id = cls.sid_userid.pop(sid, None)
-        cls.userid_sid.pop(userid, None)
+        cls.userid_sid.pop(user_id, None)
 
     @classmethod
     def __str__(cls):