fix spec:media-capabilities reference (#946)

Author: Djuffin

index.src.html

@@ -35,6 +35,7 @@
 spec:infra; type:dfn; text:enqueue
 spec:webaudio-1.1; type:interface; text:AudioWorklet
 spec:webaudio-1.1; type:interface; text:AudioBuffer
+spec:media-capabilities; type:dict-member; text:powerEfficient
 </pre>
 
 <pre class='anchors'>