README / package description: broken hyperlink to advanced usage documentation

[jayaddison]

The hyperlink on the phrase Advanced Usage in this line of the README seems to navigate to an HTTP 404:

httpx/README.md

Line 112 in 26d48e0

For more advanced topics, see the [Advanced Usage](https://www.python-httpx.org/advanced/) section, the [async support](https://www.python-httpx.org/async/) section, or the [HTTP/2](https://www.python-httpx.org/http2/) section.

I'm not sure what to suggest as a fix - either for https://www.python-httpx.org/advanced/ to display an index of advanced topics, or perhaps to choose an alternate destination for that hyperlink.