Update README.md python example (#4639)

README.md

@@ -119,7 +119,7 @@ from autogen_agentchat.agents import AssistantAgent
 from autogen_agentchat.ui import Console
 from autogen_agentchat.conditions import TextMentionTermination
 from autogen_agentchat.teams import RoundRobinGroupChat
-from autogen_ext.models import OpenAIChatCompletionClient
+from autogen_ext.models.openai import OpenAIChatCompletionClient
 
 # Define a tool
 async def get_weather(city: str) -> str: