Config Option: 'type_labels' expects JSON, not YAML map

Despite the example `type_labels` mapping showing a serialized JSON 1-liner, I didn't process the idea of embedding JSON config inside YAML. This led to a somewhat obscure error when my workflow tried to execute:

```
Invalid workflow file
The workflow is not valid. .github/workflows/auto-label.yml (Line: 32, Col: 13): A mapping was not expected
```

I have a pending fix in https://github.com/GoogleCloudPlatform/emblem/pull/391.

More explicit documentation that JSON string is expected (such as a complete example showing use of that configuration), or support for a YAML maps would have made my experience as a first-time user easier.

Start with .github/workflows/auto-label.yml around line 32 and review the type_labels configuration described in the issue; check the pending fix in GoogleCloudPlatform/emblem pull request 391 for context. Done means the documentation includes a complete example that clearly shows type_labels must be a JSON string, or the supported YAML-map behavior is explicitly documented.

Written by the indexing model from the issue text.