Fix 'this' while switching subtitle tracks (#2441)

When you switch a subtitle track you'll get

`TypeError: Cannot read properties of undefined (reading 'ui')` in `captions.js`'s `captions.setup`,

Because `captions.setup` is being called without proper `this` binding.

src/js/plyr.js

@@ -958,7 +958,7 @@ class Plyr {
    */
   set currentTrack(input) {
     captions.set.call(this, input, false);
-    captions.setup();
+    captions.setup.call(this);
   }
 
   /**