Fix link to cabal docs

README.md

@@ -67,7 +67,7 @@ active-repositories: hackage.haskell.org, head.hackage.ghc.haskell.org
 ```
 
 Also see
-https://cabal.readthedocs.io/en/latest/cabal-project.html#cfg-field-active-repositories.
+https://cabal.readthedocs.io/en/3.12/cabal-project-description-file.html#cfg-field-active-repositories.
 
 `HEAD.hackage` doesn't bump the bounds of boot packages + certain other packages to avoid the busywork of bumping them. When using `HEAD.hackage`, you should use `--allow-newer` for these packages. The full list is [here](https://gitlab.haskell.org/ghc/head.hackage/-/blob/90570e1c4606c1d7d3d41797ec1b32d1b984067b/ci/MakeConstraints.hs#L40-49).