Update spec/index.html

Co-authored-by: Andy Seaborne <[email protected]>

Author: gkellogg

spec/index.html

@@ -57,7 +57,7 @@
       where the elements may be IRIs, blank nodes, or datatyped literals.
       They are used to express descriptions of resources.</li>
     <li>RDF datasets are used to organize collections of RDF graphs,
-      and comprise a default graph and zero or more named graphs.</li>
+      are comprised of a default graph and zero or more named graphs.</li>
   </ul>
 
   <p>RDF 1.2 Concepts also introduces key concepts and terminology, and discusses