Update media type registration

[afs]

Currently:
https://www.w3.org/TR/rdf12-xml/#section-MIME-Type

3.1 RDF/XML Internet Media Type, File Extension, and Macintosh File Type
Contact:
Dan Brickley

This needed updating at RDF 1.1 but it was missed.

More generally, the section doesn't follow the more detailed style of other RDF syntax registrations: Turtle example.