Update README.md

The widgets extension looks for the key 'widgets', not for the key widget:

```js
 afterAppStart: function(app) {
        // Auto start widgets when the app is loaded.
        if (app.startOptions.widgets) { // <== RIGHT HERE!
          app.core.start(app.startOptions.widgets);
        }
      }
```

So i'm updating the readme to describe it properly.
Merge pull request #251 from Pickachu/patch-1

Fix wrong key on README.md example: 'widget' for 'widgets'