onionshare: fix stem override

pkgs/applications/networking/onionshare/default.nix

···
     license = licenses.gpl3Plus;
     maintainers = with maintainers; [ lourkeur ];
   };
-  stem' = stem.overrideAttrs (_: rec {
+  stem' = stem.overridePythonAttrs (_: rec {
     version = "1.8.1";
 
     src = fetchFromGitHub {