fixed incorrect help for dig command

After the execution of "dig" command for new files, the help display the following message

```
tomb (*) Done digging test
tomb  .  your tomb is not yet ready, you need to forge a key and lock it:
tomb  .  tomb forge test.tomb.key
tomb  .  tomb lock test.tomb test.tomb.key
```
But the correct command should be
```
tomb  .  tomb lock test.tomb -k test.tomb.key
```