factorio: fix review comments

nixos/modules/services/games/factorio.nix

@@ -140,7 +140,9 @@ in
         type = types.nullOr types.path;
         default = null;
         description = lib.mdDoc ''
-        Mods settings can be changed by specifying a dat file.
+          Mods settings can be changed by specifying a dat file, in the [mod
+          settings file
+          format](https://wiki.factorio.com/Mod_settings_file_format).
         '';
       };
       game-name = mkOption {

pkgs/games/factorio/utils.nix

@@ -18,9 +18,9 @@ with lib;
           # NB: there will only ever be a single zip file in each mod derivation's output dir
           ln -s $modDrv/*.zip $out
         done
-      '' + (if modsDatFile != null then ''
+      '' + (lib.optionalString (modsDatFile != null) ''
        cp ${modsDatFile} $out/mod-settings.dat
-      '' else "");
+      '');
     };
 
     modDrv = { allRecommendedMods, allOptionalMods }: