sw-raid: rename mdadmShutdown

See https://github.com/NixOS/nixpkgs/commit/aac666e302807805154190ebe105711d585c90ee
I've forgotten to add the change.

nixos/modules/tasks/swraid.nix

···
     cp -v ${pkgs.mdadm}/lib/udev/rules.d/*.rules $out/
   '';
 
-  systemd.services.mdadmShutdown = {
+  systemd.services.mdadm-shutdown = {
     wantedBy = [ "final.target"];
     after = [ "umount.target" ];