nixos-generate-config: fix nix-env example command

An escape char is needed to prevent "$ nix" from being evaluated and
expanded to an empty string. With this change the resulting text is

  $ nix-env -qaP | grep wget

instead of

  -env -qaP | grep wget

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -466,7 +466,7 @@ $bootLoaderConfig
   # };
 
   # List packages installed in system profile. To search by name, run:
-  # $ nix-env -qaP | grep wget
+  # \$ nix-env -qaP | grep wget
   # environment.systemPackages = with pkgs; [
   #   wget
   # ];