[MMSEM-Vocabulary] new link for MPEG-7 ontology

Hi Michael,

I've put a new link for Hunter's MPEG-7 ontology into the vocabulary 
page. A more recent version at [1] has fixed the xsd:nil mistake so I 
also removed that comment. The ontology validates using WonderWeb OWL 
Ontology Validator.

Hope this is the correct procedure! :-)

Regards,
Suzanne

[1] http://metadata.net/mpeg7/