Please update the "Nixpkgs" name and link on homepage

[Bjørn Forsman]

In http://microscheme.org/download, it says

"[...] and Nixpkgs package managers"

(where "Nixpkgs" is a link)

Instead of "Nixpkgs" it should say "Nix", and instead of the current URL, it should say https://nixos.org/nix/.

Thanks.

[Ryan Suchocki]

Great, Done!