README: sync package count with https://search.nixos.org/packages (#399333)

</p>

[Nixpkgs](https://github.com/nixos/nixpkgs) is a collection of over

100,000 software packages that can be installed with the

120,000 software packages that can be installed with the

[Nix](https://nixos.org/nix/) package manager. It also implements

[NixOS](https://nixos.org/nixos/), a purely-functional Linux distribution.