rtorrent: fix updateScript eval

Without the change updater run fails as:

    error: function 'anonymous lambda' called with unexpected argument 'tagPrefix'

pkgs/by-name/rt/rtorrent/package.nix

@@ -60,7 +60,7 @@ stdenv.mkDerivation rec {
   ];
 
   passthru = {
-    updateScript = gitUpdater { tagPrefix = "v"; };
+    updateScript = gitUpdater { rev-prefix = "v"; };
     tests = {
       inherit (nixosTests) rtorrent;
     };