Fix for Parallels builder ID in vagrant post-processor

3a68c8a changed builder ID and it caused an error while `parallels-iso` building:
```
==> parallels-iso: Running post-processor: vagrant
Build 'parallels-iso' errored: 1 error(s) occurred:

* Post-processor failed: Unknown artifact type, can't build box: packer.parallels
```

This pull-request contains a fix for this error