home-cursor: update URL to Freedesktop.org specs

The old one returns a `404 - Page not found` error.

Located the content via archive.org¹ and a quick search yielded the
current URL for the spec.

¹: https://web.archive.org/web/20161024065530/https://specifications.freedesktop.org/icon-theme-spec/latest/ar01s03.html

modules/config/home-cursor.nix

@@ -203,7 +203,7 @@ in
         (mkIf cfg.dotIcons.enable {
           # Add symlink of cursor icon directory to $HOME/.icons, needed for
           # backwards compatibility with some applications. See:
-          # https://specifications.freedesktop.org/icon-theme-spec/latest/ar01s03.html
+          # https://specifications.freedesktop.org/icon-theme/latest/#directory_layout
           home.file.".icons/default/index.theme".source =
             "${defaultIndexThemePackage}/share/icons/default/index.theme";
           home.file.".icons/${cfg.name}".source = "${cfg.package}/share/icons/${cfg.name}";