Update vocabulary mapping definition to allow any IRI, not just an absolute IRI.

For w3c/json-ld-syntax#72.

Author: gkellogg

common/terms.html

@@ -335,5 +335,5 @@
     A <a>value object</a> is a <a>dictionary</a> that has an <code>@value</code> <a>member</a>.</dd>
   <dt><dfn>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>absolute IRI</a> or <code>null</code>.</dd>
+    whose value MUST be an <a>IRI</a> or <code>null</code>.</dd>
 </dl>