Fix PEP 508 link in preview doc specifying_dependencies (#4158)

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## Summary

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

Fix PEP 508 link.

## Test Plan

See preview
https://github.com/SigureMo/uv/blob/fix-pep508-link-in-preview-doc/docs/specifying_dependencies.md

docs/specifying_dependencies.md

@@ -14,7 +14,7 @@ different index, etc.
 ## `project.dependencies`
 
 The `project.dependencies` table represents the dependencies that are used when uploading to PyPI or
-building a wheel. Individual dependencies are specified using [PEP 508](#PEP 508), and the table as
+building a wheel. Individual dependencies are specified using [PEP 508](#pep-508), and the table as
 a whole follows the [PEP 621](https://packaging.python.org/en/latest/specifications/pyproject-toml/)
 standard.