Restore SHOULD NOT from RDF 1.1

Author: afs

spec/index.html

@@ -1321,8 +1321,8 @@ <h3>Datatype IRIs</h3>
       Any literal typed with a datatype not handled by an RDF implementation
       is treated just like an unknown IRI, i.e., as referring to an unknown thing.
       Applications MAY give a warning message if they are unable to determine the
-      referent of an IRI used in a typed literal. RDF implementations SHOULD
-      not reject a literal with an unknown datatype as either a syntactic or
+      referent of an IRI used in a typed literal. RDF implementations SHOULD NOT
+      reject a literal with an unknown datatype as either a syntactic or
       semantic error.<p>
 
     <p>Other specifications MAY impose additional constraints on