Update Step 3 in nixos module guide (#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;
      };