Update Storage/FileSystemStorage.php

Fixing `resolveUri`.

Following configuration:

```yaml
uri_prefix:           /
upload_destination:   media/product/files
```

Returned `/files/acme.png` for file named `acme.png`, when it should've returned `/media/product/files/acme.png`.

I have no idea what this $parts line was for, but it did break a very important part of this bundle!