docs: change variable declaration to modern syntax (#2692)

Issue #2544

docs/tutorials/basic-usage.md

@@ -32,7 +32,7 @@ Basic usage of Shaka Player is very easy:
 ```js
 // myapp.js
 
-var manifestUri =
+const manifestUri =
     'https://storage.googleapis.com/shaka-demo-assets/angel-one/dash.mpd';
 
 function initApp() {
@@ -51,8 +51,8 @@ function initApp() {
 
 function initPlayer() {
   // Create a Player instance.
-  var video = document.getElementById('video');
-  var player = new shaka.Player(video);
+  const video = document.getElementById('video');
+  const player = new shaka.Player(video);
 
   // Attach player to the window to make it easy to access in the JS console.
   window.player = player;