Update index.html

Apply changes from erratum, adding a @context statement for example for Tombstone (example 60 at time of writing), see issue #441 for discussion.

Author: evanp

vocabulary/index.html

@@ -2751,6 +2751,7 @@ <h2>Object and Link Types</h2>
             <td rowspan="4">
 <div id="ex184b-jsonld" style="display: block;">
 <pre class="example highlight json">{
+  "@context": "https://www.w3.org/ns/activitystreams",
   "type": "OrderedCollection",
   "totalItems": 3,
   "name": "Vacation photos 2016",