update example to use navbar-nav class

if you try to render the nav without this class it renders each item on its own line.  this change means they are rendered inline as expected.

NavBar.php

@@ -27,6 +27,7 @@ use yii\helpers\ArrayHelper;
  *         ['label' => 'Home', 'url' => ['/site/index']],
  *         ['label' => 'About', 'url' => ['/site/about']],
  *     ],
+ *     'options' => ['class' => 'navbar-nav'],
  * ]);
  * NavBar::end();
  * ```