nixos/sysupdate: fix example text

The example systemd-sysupdate transfer name has a ".conf" suffix,
although the files on the final system are already appended with this
suffix, so the file ends up being "transfer-name.conf.conf". Remove the
suffix in the example so that users will get a transfer filename they
expect.

nixos/modules/system/boot/systemd/sysupdate.nix

@@ -71,7 +71,7 @@ in
       type = with lib.types; attrsOf format.type;
       default = { };
       example = {
-        "10-uki.conf" = {
+        "10-uki" = {
           Transfer = {
             ProtectVersion = "%A";
           };