Use setAttributeNode instead of appendChild to set attribute

Co-authored-by: Weston Ruter <westonruter@google.com>

Author: ediamin

src/Optimizer/Transformer/ServerSideRendering.php

@@ -1079,7 +1079,7 @@ private function ssrAmpAudio(Document $document, Element $element)
         $audio    = $document->createElement(Tag::AUDIO);
         $controls = $document->createAttribute(Attribute::CONTROLS);
 
-        $audio->appendChild($controls);
+        $audio->setAttributeNode($controls);
         $element->appendChild($audio);
     }
 }