fixed namespace error in BLD

In the media-type registration appendix of BLD, I had put the namespace
down as "http://www.w3.org/ns/rif".  I thought we decided to adopt that,
but perhaps I was misremembering a meeting.  (Perhaps an OWL meeting...)
Anyway, I've changed it to "http://www.w3.org/2007/rif#" to match the
rest of the documents.

     -- Sandro