sw-raid: rename mdadmShutdown

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

Changed files
+1 -1
nixos
modules
tasks
+1 -1
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" ];