make items optional in SetQueueOptions definition

[nchase]

the documentation seems to have some internal conflict about whether or not items is necessary, but in usage it appears to be optional.

Here, items is not defined as optional for MusicKit.SetQueueOptions: https://developer.apple.com/documentation/musickitjs/musickit/setqueueoptions

But here, setQueue is called without items: https://developer.apple.com/documentation/musickitjs/accessing_musickit_features_using_javascript#3004833

^^ If that URL doesn't work, this is the example:

// "You can initialize a playback queue by using the setQueue method,
//  either with individual media items, or with an album or playlist."
//
// "To set a queue using an album's URL:"

let music = MusicKit.getInstance();
let url = 'https://itunes.apple.com/us/album/hamilton-original-broadway-cast-recording/1025210938';
music.setQueue({ url: url }).then(function(queue) {
  // Queue is instantiated and set on music player.
});

And in usage it seems to work.