Vocabulary Plugin error: [2021/07/22 09:27:12 unsupported document type surface.v1.Model]

### Summary:

Error message is returned by the vocabulary plugin when running the tests listed in the gnostic readme.

### Steps to reproduce/observations:

When running step 8 in this project's README:

```
gnostic examples/v2.0/json/petstore.json --vocabulary_out=.
```

the following error message is displayed:

```
2021/07/22 09:27:12 unsupported document type surface.v1.Model
```

This make is look like the step has failed, but upon checking the output folder the expected `vocabulary.json` file etc are created. What is not clear is whether this is a partial failure and some of the output data is missing in those files.

### Further info

Tested on a Mac (v11.5) and Windows (Version 10.0.19042.1083) in case this was an installation problem on my device. Same error message returned on both.

Repo at commit 0320d74 at the time the error was experienced.