nixos/sudo: Update assertion message

  ###### implementation

  config = mkIf cfg.enable {

    assertions = [

      { assertion = cfg.package.pname != "sudo-rs";

        message = "The NixOS `sudo` module does not work with `sudo-rs` yet."; }

    ];

    assertions = [ {

      assertion = cfg.package.pname != "sudo-rs";

      message = ''

        NixOS' `sudo` module does not support `sudo-rs`; see `security.sudo-rs` instead.

      '';

    } ];

    security.sudo.extraRules =

      let