core-vocab.html uses shapeToSpec to generate the HTML tables from the CommonProperties-shape.ttl file. The namespace for FOAF is:
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
But that is not a URI that supports an RDF representation of FOAF. In ResouceShape <#Person>, setting oslc:describes to foaf:Person causes ReSpec to fail.