Docs: fix 404ing /advanced/#<anchor> links -> /advanced/clients/#<anchor>

[nick2i]

Summary

A couple of the documentation links in httpx/_client.py docstrings were out of date and led to 404's:
https://www.python-httpx.org/advanced/#request-instances
https://www.python-httpx.org/advanced/#merging-of-configuration

These have been updated to:

https://www.python-httpx.org/advanced/clients/#request-instances
https://www.python-httpx.org/advanced/clients/#merging-of-configuration

I hope this doesn't require a prior discussion 😄

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[lovelydinosaur]

Thanks ☺️ - is this ready to leave draft?

[nick2i]

Thanks ☺️ - is this ready to leave draft?

Yes, sorry, I noticed a few other similarly broken links and will put up a separate PR