update docs

packages/artplayer-vitepress/docs/library/dash.md

@@ -9,13 +9,11 @@
 ```js{18-21,25}
 function playMpd(video, url, art) {
     if (dashjs.supportsMediaSource()) {
+        if (art.dash) art.dash.destroy();
         const dash = dashjs.MediaPlayer().create();
         dash.initialize(video, url, art.option.autoplay);
-
-        // optional
         art.dash = dash; 
-        art.once('url', () => dash.destroy());
-        art.once('destroy', () => dash.destroy());
+        art.on('destroy', () => dash.destroy());
     } else {
         art.notice.show = 'Unsupported playback format: mpd';
     }