Update README.md python example

Update the Python example in the readme to import OpenAIChatCompletionClient from the new path

Currently, the example fails with this error:
ImportError: cannot import name 'OpenAIChatCompletionClient' from 'autogen_ext.models' (unknown location)

This change relates to: microsoft#4601

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: