Update definition for _vocabulary mapping_ to specifically allow compact IRI and term.

For w3c/json-ld-syntax#184.

Author: gkellogg

common/terms.html

@@ -350,6 +350,6 @@
     A <a>value object</a> is a <a>map</a> that has an <code>@value</code> <a>entry</a>.</dd>
   <dt><dfn data-cite="JSON-LD11#dfn-vocabulary-mapping" class="preserve">vocabulary mapping</dfn></dt><dd>
     The vocabulary mapping is set in the <a>context</a> using the <code>@vocab</code> key
-    whose value MUST be an <a>IRI</a> or <code>null</code>.</dd>
+    whose value MUST be an <a>IRI</a>, a <a>compact IRI</a>, a <a>term</a>, or <code>null</code>.</dd>
 </dl>
 </section>