Fix read/write get/set mixup in help command

The `trousseau help get` and `trousseau help set` texts are mixed up.

```bash
% trousseau help set
NAME:
   set - Set a key value pair in the encrypted data store

USAGE:
   command set [command options] [arguments...]

OPTIONS:
   --file, -f   Write key's value to provided file
```

The option `--file` is actually getting the value from a file when using `set`, and vice versa with `get`.