Fix NixOS install instructions
Fix NixOS install instructions

Using `nix-env` you have two options:
`nix-env -i rkt` or `nix-env -iA nixos.rkt`

Using `nix-env -iA rkt` will usually not work.

On NixOS however you also want general virtualization dependencies.
Therefore enabling the rkt virtualisation is the right way to
set up everything correctly.

joshix/rkt,kinvolk/rkt,joshix/rkt,coreos/rkt,joshix/rkt,joshix/rkt,coreos/rkt,kinvolk/rkt,rkt/rkt,kinvolk/rkt,coreos/rkt,rkt/rkt,rkt/rkt,rkt/rkt,kinvolk/rkt