nixos/nitter: update example configuration file URL

It was moved.

nixos/modules/services/misc/nitter.nix

···
           Add settings here to override NixOS module generated settings.
 
           Check the official repository for the available settings:
-          https://github.com/zedeus/nitter/blob/master/nitter.conf
+          https://github.com/zedeus/nitter/blob/master/nitter.example.conf
         '';
       };