docs: change promise then to await (#2693)

Closes #2544

docs/tutorials/basic-usage.md

@@ -49,7 +49,7 @@ function initApp() {
   }
 }
 
-function initPlayer() {
+async function initPlayer() {
   // Create a Player instance.
   const video = document.getElementById('video');
   const player = new shaka.Player(video);
@@ -62,10 +62,14 @@ function initPlayer() {
 
   // Try to load a manifest.
   // This is an asynchronous process.
-  player.load(manifestUri).then(function() {
+  try {
+    await player.load(manifestUri);
     // This runs if the asynchronous load is successful.
     console.log('The video has now been loaded!');
-  }).catch(onError);  // onError is executed if the asynchronous load fails.
+  } catch (e) {
+    // onError is executed if the asynchronous load fails.
+    onError(e);
+  }
 }
 
 function onErrorEvent(event) {