Editorial: Align with Web IDL specification (#351)

Author: autokagami

media-source-respec.html

@@ -4621,7 +4621,7 @@ 
       <pre class="idl">
         [Exposed=(Window,DedicatedWorker)]
         interface BufferedChangeEvent : Event {
-          constructor(DOMString type, BufferedChangeEventInit eventInitDict);
+          constructor(DOMString type, optional BufferedChangeEventInit eventInitDict = {});
 
           [SameObject] readonly attribute TimeRanges addedRanges;
           [SameObject] readonly attribute TimeRanges removedRanges;