Update the license details in the plugin bootstrap file, which was missed in #123

Author: stevegrunwell

woocommerce-custom-orders-table.php

@@ -6,8 +6,8 @@
  * Version:              1.0.0-rc2
  * Author:               Liquid Web
  * Author URI:           https://www.liquidweb.com
- * License:              GPL2
- * License URI:          https://www.gnu.org/licenses/gpl-2.0.html
+ * License:              GPLv3 or later
+ * License URI:          https://www.gnu.org/licenses/gpl-3.0.html
  *
  * WC requires at least: 3.5.1
  * WC tested up to:      3.6.4