Fix the bug to enable the demo_mode

Author: yi xiang

src/agentlab/ui_assistant.py

@@ -9,7 +9,7 @@
 
 def make_exp_args(agent_args: AgentArgs, start_url="https://www.google.com"):
     try:
-        agent_args.flags.action.demo_mode = "default"
+        agent_args.flags.action.action_set.demo_mode = "default"
     except AttributeError:
         pass