fix left over closing backticks

Collapse.php

@@ -53,8 +53,6 @@ class Collapse extends Widget
      * - content: string, required, the content (HTML) of the group
      * - options: array, optional, the HTML attributes of the group
      * - contentOptions: optional, the HTML attributes of the group's content
-     *
-     * ```
      */
     public $items = [];