Add 'RDFS' in comment

dcat/index.html

@@ -273,7 +273,7 @@ <h3>DCAT scope</h3>
         Along with the rest of <a href="#vocabulary-overview"></a>, this diagram is <b>non-normative</b>.
         Furthermore, while the diagram uses UML-style class notation it should be interpreted following the usual RDF open-world assumptions around the presence/absence of properties, relationships, and their cardinality.
         The properties shown in each class reflect those specified in the descriptions of classes in <a href="#vocabulary-specification"></a>.
-        Open arrow-heads indicate <i>sub-class-of</i> relationships, rather than object-oriented generalization. 
+        Open arrow-heads indicate RDFS <i>sub-class-of</i> relationships (not object-oriented generalization). 
         To assist in understanding the full scope of each class, all applicable properties are copied down from each '::super-class'.
         Cardinalities are shown in a few places to reinforce expectations, but these are not axiomatized or enforced in any way by this (normative) recommendation.
       </p>