encrypted devices: provide working example

nixos/modules/tasks/encrypted-devices.nix

···
 
       keyFile = mkOption {
         default = null;
-        example = "/root/.swapkey";
+        example = "/mnt-root/root/.swapkey";
         type = types.nullOr types.str;
         description = "File system location of keyfile. This unlocks the drive after the root has been mounted to <literal>/mnt-root</literal>.";
       };