add missing translation en.vagrant.commands.status.stopping

VirtualBox got into a limbo mode, as a result, running `vagrant status` reported:

```
$ vagrant status                                                                                                                              *[feature/remove-legacy-api]
Current machine states:

default                   stopping (virtualbox)

translation missing: en.vagrant.commands.status.stopping
```

To satisfy my lack of creativity, please suggest a better text. Much obliged :)