Change gist URL to the official MCP quickstart repo

Author: Alex Akimov

docs/docs/develop/build-client.mdx

@@ -279,7 +279,7 @@ if __name__ == "__main__":
     asyncio.run(main())
 ```
 
-You can find the complete `client.py` file [here.](https://gist.github.com/zckly/f3f28ea731e096e53b39b47bf0a2d4b1)
+You can find the complete `client.py` file [here](https://github.com/modelcontextprotocol/quickstart-resources/blob/main/mcp-client-python/client.py).
 
 ## Key Components Explained