Merge pull request #12986 from Ericson2314/patch-1

doc: Fix URL to ghc-paths patch

doc/haskell-users-guide.md

···
 the library can't possible know that path when it's compiled, because the path
 GHC considers its own is determined only much later, when the user configures
 it through `ghcWithPackages`. So we [patched
-ghc-paths](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/ghc-paths-nix.patch)
+ghc-paths](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/patches/ghc-paths-nix.patch)
 to return the paths found in those environment variables at run-time rather
 than trying to guess them at compile-time.