apps:destroy should have a more explicit help message

Running

```
deis apps:destroy
```

yields the help message because syntax is not correct. The `-a` flag is missing. However, the help message is not very helpful, it should at least mention this flag.

```
$ deis apps:destroy italic-nuthatch
Usage: deis apps:destroy [options]
```

It's not a big deal, but IMO it would improve ergonomy a little bit