Update Step 3 in nixos module guide (kmonad#749)

* Update NixOS module in installation.md

Step 3 on nixos module guide, will not work as it is stated right now.

* Update NixOS module gude in installation.md

Step 3 on nixos module guide, will not work as it is stated right now.

doc/installation.md

@@ -477,9 +477,16 @@ binary release of kmonad and packages it in the nix-store:
 3. Configure the module:
 
  ``` nix
- services.kmonad = {
- enable = true; # disable to not run kmonad at startup
- configfiles = [ /path/to/config.kbd ];
+ services.kmonad = {
+ enable = true;
+ keyboards = {
+ { any name for your keyboard } = {
+ device = "/dev/input/by-id/{ your keyboard id that usually ends with -kbd }";
+ config = ''
+ { content of config.kbd here }
+ '';
+ };
+ }; 
  # Modify the following line if you copied nixos-module.nix elsewhere or if you want to use the derivation described above
  # package = import /pack/to/kmonad.nix;
  };