docs: Fixed broken link in 'Displaying a Now Playing Card' lesson.

<p>The Now Playing card shows up after {@link android.media.session.MediaSession#setActive(boolean) setActive(true)}

is called, if the session is the highest priority session in the system. Also, note that your app

must request the audio focus, as described in <a href="{@docRoot}training/managing-audio/audio-focus">

Managing Audio Focus</a>.</p>

must request the audio focus, as described in

<a href="{@docRoot}training/managing-audio/audio-focus.html">Managing Audio Focus</a>.</p>

<pre>

private void handlePlayRequest() {