Remove mention of blank node identifier from VC Graph definition.

Author: msporny

index.html

@@ -2261,9 +2261,9 @@ <h4>Verifiable Credential Graphs</h4>
 credential</a> with data from another, such as when the `credentialSubject.id`
 is the same in both <a>verifiable credentials</a>, but the object might contain
 objects of the "Jane Doe" form described in the previous paragraph. It is
-important to not merge objects that use document-relative identifiers, such as
-blank node identifiers, and to only perform merges when global identifiers, such
-as URLs, are used.
+important to not merge objects that seem to have similar properties but do not
+contain an `id` property that uses a global identifier, such
+as a URL.
           </p>
         </section>