nixos/printers: fix example for hardware.printers.ensurePrinters.*.model
nixos/printers: fix example for hardware.printers.ensurePrinters.*.model

It's `lib.versions`, not `lib.version`. Also I'm really sure that it's
supposed to be the current version of Gutenprint, not Cups, as thats
what `lpinfo -m` says on my system.

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