clarify text

re-reading this, I realized it could be surprising:
the predicate position of a triple is a "context where IRIs are expected",
and so this text could read as "don't use abbreviated forms in the predicate position",
which is of course not how it was intended.

Author: pchampin

spec/index.html

@@ -305,7 +305,7 @@ <h3>RDF Vocabularies and Namespace IRIs</h3>
       the IRI <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral</code>
       would be abbreviated as <code>rdf:XMLLiteral</code>.
       Note however that these abbreviations are <em>not</em> valid IRIs (or only coincidentally),
-      and must not be used in contexts where IRIs are expected.
+      and must not be used in syntactic contexts where IRIs are expected.
       Namespace IRIs and namespace prefixes are <em>not</em> a formal part of the
       RDF data model. They are merely a syntactic convenience for
       abbreviating IRIs.</p>