Update to MAPv4 public draft

This should be in line with the draft at: https://docs.google.com/document/d/1Jh8ULpw0jb8kyxV-Ygw9U0n-XqXkM6_V3jxfmnkebqo/edit#
MAPv4 discussion - should we just change this to `:genre` or `:hasType`? (MAPv3.1 JSON-LD is `sourceResource.specType`.)