doc: point out that nixos-21.05 has gnuradio 3.9
doc: point out that nixos-21.05 has gnuradio 3.9

Reading the release notes I got the impression that the latest (and
default) was GR3.8, but it is in fact 3.9. Make that more obvioius.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs