initial prefixmap seems to be wrong format

[balhoff]

In the linkml workshop, I ran make setup after creating the project. The file it made at prefixmap/linkml_tutorial.yaml is not YAML, it’s JSON. And doesn’t seem like it came from the prefixmap command because it has a nested object like:

 "NamedThing": {
      "@id": "schema:Thing"
   }