nixos/factorio: correct extraSettings example

"admins" isn't a server setting, so this example wouldn't actually work.
"max_players" is the first thing I saw in the [example server settings
file][1] that hasn't already been encoded in the options.

[1]: https://github.com/wube/factorio-data/blob/master/server-settings.example.json

nixos/modules/services/games/factorio.nix

@@ -177,7 +177,7 @@ in
       extraSettings = lib.mkOption {
         type = lib.types.attrs;
         default = {};
-        example = { admins = [ "username" ];};
+        example = { max_players = 64; };
         description = ''
           Extra game configuration that will go into server-settings.json
         '';