Update spec/index.html

Co-authored-by: Niklas Lindström <lindstream@gmail.com>

Author: afs

spec/index.html

@@ -288,7 +288,8 @@ <h3>The Referent of an IRI</h3>
       unique identifiers in a graph data model that describes resources.
       However, those interactions are critical to the concept of
       [[[LINKED-DATA]]], [[LINKED-DATA]],
-      which uses the RDF <a>abstract syntax</a> and serialization formats.</p>
+      which uses the RDF <a>abstract syntax</a> and <a>concrete RDF syntaxes</a>,
+	  the latter also referred to as serialization formats.</p>
   </section>
 
   <section id="vocabularies">