doc: update spec to indicate objects can be empty

Fixes #1105

Author: johnbartholomew

doc/ref/spec.html

@@ -537,7 +537,7 @@ <h2 id="abstract_syntax">Abstract Syntax</h2>
           <td>::=</td>
           <td>
             <!-- object -->
-            <i>member</i> { <code>,</code> <i>member</i> } [ <code>,</code> ]
+            { <i>member</i> <code>,</code> } [ <i>member</i> ]
           </td>
         </tr>