Fix Creating BCContract through REST API

We are testing the server distribution by following the tutorial https://doc.nuxeo.com/nxdoc/discover-nuxeo-platform-apis/.

1. Run a 9.3 server with the addon nuxeo-platform-getting-started.
2. Install the JS client as explained at https://doc.nuxeo.com/nxdoc/setting-up-your-javascript-environment/#setting-up-the-javascript-environment.
3. Follow the Creating a Document steps at https://doc.nuxeo.com/nxdoc/manipulating-documents/.

=> You get the error attached.