Update JSON product to use correct download PURL cl.md

This is, if I remember correctly the consequence of a design decision for the PURL system.

While the PURL defined by ODK etc is http://purl.obolibrary.org/obo/cl.json, the PURL system only recognises a tiny number of defaults _above_ the ontology namespace (including cl.owl and cl.obo), but cl.json is not part of it.

We could argue that we should also add cl.json to this list.

Author: matentzn

ontology/cl.md

@@ -45,7 +45,7 @@ products:
   derived_from: cl.owl
   description: Complete ontology, plus inter-ontology axioms, and imports modules merged in
   format: obo
-- id: cl.json
+- id: cl/cl.json
   title: CL OBOGraph-JSON format edition
   derived_from: cl.owl
   description: Complete ontology, plus inter-ontology axioms, and imports modules merged in