`config.json` expects array even for single value

[gramian]

When using the label_property it is required that JSON array is passed in the config.json, even when only a single property URI is passed. If this is wanted behavior a note in the README would be sufficient.

Discovered using the fork https://github.com/fatestigma/ontology-visualization on Ubuntu 20.04, with Python 3.8.10, and python3-rdflib 4.2.2-5.