Minor test case fix

Somebody pointed me at
  http://www.w3.org/2000/10/rdf-tests/rdfcore/unrecognised-xml-attributes/test002.rdf

and found the use of "in the xml namespace" misleading, it isn't
related to namespaces but attributes starting with 'x' 'm' 'l'.  This
is a minor wording fix, no tracking issue needed - just recording.

Dave