nixos/zfs: fix autoScrub.enable description

This was previously "Whether to enable Enables periodic scrubbing of
ZFS pools.."

nixos/modules/tasks/filesystems/zfs.nix

···
     };
 
     services.zfs.autoScrub = {
-      enable = mkEnableOption "Enables periodic scrubbing of ZFS pools.";
+      enable = mkEnableOption "periodic scrubbing of ZFS pools";
 
       interval = mkOption {
         default = "Sun, 02:00";