Fix link to publish guide in README (#8720)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

The README contains a link to the publish guide
(https://docs.astral.sh/uv/guides/publish.md) that gives an error 404.
When navigating to that page via the side menu, the correct link seems
to be (https://docs.astral.sh/uv/guides/publish/).

## Test Plan

<!-- How was it tested? -->

Documentation change, therefore manual testing by clicking on the link.

README.md

@@ -103,7 +103,7 @@ All checks passed!
 See the [project documentation](https://docs.astral.sh/uv/guides/projects/) to get started.
 
 uv also supports building and publishing projects, even if they're not managed with uv. See the
-[publish guide](https://docs.astral.sh/uv/guides/publish.md) to learn more.
+[publish guide](https://docs.astral.sh/uv/guides/publish/) to learn more.
 
 ### Tool management