gnomeExtensions.tilingGnome: remove unstable in pname

We want the pname to match a potential automatic packaging (if the extension is updated to support a more recent
GNOME version) as closely as possible.

pkgs/desktops/gnome/extensions/tilingnome/default.nix

···
 { stdenv, lib, fetchFromGitHub, glib, gnome }:
 
 stdenv.mkDerivation rec {
-  pname = "gnome-shell-extension-tilingnome-unstable";
+  pname = "gnome-shell-extension-tilingnome";
   version = "unstable-2019-09-19";
 
   src = fetchFromGitHub {