Editorial: remove some sandbox attribute copypasta

Author: annevk

source

@@ -30164,7 +30164,7 @@ interface <dfn>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
   <p>When an <code>iframe</code> element's <code data-x="attr-iframe-sandbox">sandbox</code>
   attribute is removed while it has a non-null <span>nested browsing context</span>, the user agent
   must empty the <code>iframe</code> element's <span>nested browsing context</span>'s
-  <span><code>iframe</code> sandboxing flag set</span> as the output.</p>
+  <span><code>iframe</code> sandboxing flag set</span>.</p>
 
   </div>