Add missing li, even though it is missing from the place where I copied this step

Author: domfarolino

dom.bs

@@ -2947,6 +2947,7 @@ to do these things asynchronously, however.
 
    <li><p>Run <a>assign slottables for a tree</a> with <var>node</var>.
   </ol>
+ </li>
 
  <!-- Start insertion-related bookkeeping steps -->
  <li>