Editorial: Fix some minor wording issues for media events

source

@@ -39689,7 +39689,7 @@ dictionary <dfn dictionary>TrackEventInit</dfn> : <span>EventInit</span> {
      <td><code>Event</code>
 
      <td>An error occurs while fetching the <span>media data</span> or the type of the resource
-     is not supported media format.
+     is not a supported media format.
 
      <td><code data-x="dom-media-error">error</code> is an object with the code <code
      data-x="dom-MediaError-MEDIA_ERR_NETWORK">MEDIA_ERR_NETWORK</code> or higher. <code
@@ -39932,7 +39932,7 @@ dictionary <dfn dictionary>TrackEventInit</dfn> : <span>EventInit</span> {
   </table>
 
 
-  <p>The following event fires on <code>source</code> element:</p>
+  <p>The following event fires on <code>source</code> elements:</p>
 
   <table>
 
@@ -39953,7 +39953,7 @@ dictionary <dfn dictionary>TrackEventInit</dfn> : <span>EventInit</span> {
      <td><code>Event</code>
 
      <td>An error occurs while fetching the <span>media data</span> or the type of the resource
-     is not supported media format.
+     is not a supported media format.
 
   </table>