fix(docs): fix link to advanced python httpx docs (openai#1499)

Author: stainless-app[bot]

README.md

@@ -560,7 +560,7 @@ You can directly override the [httpx client](https://www.python-httpx.org/api/#c
 
 - Support for proxies
 - Custom transports
-- Additional [advanced](https://www.python-httpx.org/advanced/#client-instances) functionality
+- Additional [advanced](https://www.python-httpx.org/advanced/clients/) functionality
 
 ```python
 from openai import OpenAI, DefaultHttpxClient