nixos: zfs: correct the networking.hostId error message

Users were confused that the error message said config.networking.hostId, and indeed that did nothing to fix their problem.

Update the error message to specify the option they should actually set.

nixos/modules/tasks/filesystems/zfs.nix

@@ -268,7 +268,7 @@ in
       assertions = [
         {
           assertion = config.networking.hostId != null;
-          message = "ZFS requires config.networking.hostId to be set";
+          message = "ZFS requires networking.hostId to be set";
         }
         {
           assertion = !cfgZfs.forceImportAll || cfgZfs.forceImportRoot;