Minimal Example Not Working

It would be nice if this minimal example was updated to work.
```
➜ NIX_PATH=nixpkgs=nixpkgs nix-build --no-link

error: attribute 'ghc864' missing

at /home/solomon/Development/Nix/static-haskell-nix/default.nix:35:27:

34|
35| normalHaskellPackages = pkgs.haskell.packages.${compiler};
| ^
36|
(use '--show-trace' to show detailed location information)
```