docs: update link in README (#300)

Author: jenstroeger

README.md

@@ -238,7 +238,7 @@ The build process in this repository follows the requirements in the [SLSA frame
 - Source dist (tarball)
 - SBOM (CycloneDx format)
 - HTML Docs
-- A [UNIX epoch](https://en.wikipedia.org/wiki/Unix_time) timestamp file of the build time for [reproducible builds](https://flit.pypa.io/en/latest/reproducible.html)
+- A [UNIX epoch](https://en.wikipedia.org/wiki/Unix_time) timestamp file of the build time for [reproducible builds](https://reproducible-builds.org/)
 
 To verify the artifact using the provenance follow the instructions in the [SLSA verifier](https://github.com/slsa-framework/slsa-verifier) project to install the verifier tool. After downloading the artifacts and provenance, verify each artifact individually, e.g.,: