VocBench export JSON-LD file contains an array instead of an object

VocBench JSON-LD file export contains:
```json
[
  {
    "@graph": []
  }
]
```

But should ideally be:
```json
{
  "@graph": []
}
```

Some libraries don't support parsing arrays of graph(s) like seen in this issue: https://github.com/datafoodconsortium/connector-php/issues/1.

As this is a valid JSON-LD file, I assume it's up to the library to support that.