Fix error in "get a known shadow root" algorithm (w3c#1726)

Author: lutien

index.html

@@ -4446,7 +4446,7 @@ <h3 id=shadow-root>Shadow Roots</h3>
  <ol class="algorithm">
   <li>If not <a>node reference is known</a> with <a>current session</a>,
   <a>current browsing context</a>, and <var>reference</var> return
-  <a>error</a> with <a>error code</a> <a>no such element</a>.
+  <a>error</a> with <a>error code</a> <a>no such shadow root</a>.
 
   <li>Let <var>node</var> be the result of <a>get a node</a> with
   <a>current session</a>, <a>current browsing context</a>, and