Fix for issue #383 & Support for immediate cancellation of servers

It's correct in isolation, but the description doesn't line up with the other descriptions below it. Here's how it should look:
```python
  --immediate        Cancels the server immediately (instead of on the billing
                       anniversary)
  --comment=COMMENT  An optional comment to add to the cancellation ticket
  --reason=REASON    An optional cancellation reason. See cancel-reasons for a
                       list of available options
```

If that doesn't make sense it's not a big deal. I can fix it up later if you want.