Fix links to Developer API Reference

sample/billing/ListPlans.php

@@ -3,7 +3,7 @@
 // # Get List of Plan Sample
 //
 // This sample code demonstrate how you can get a list of billing plan, as documented here at:
-// https://developer.paypal.com/webapps/developer/docs/api/#list-plans
+// https://developer.paypal.com/docs/api/#list-plans
 // API used: /v1/payments/billing-plans
 
 // Retrieving the Plan object from Create Plan Sample to demonstrate the List
@@ -16,7 +16,7 @@ try {
     // Get the list of all plans
     // You can modify different params to change the return list.
     // The explanation about each pagination information could be found here
-    // at https://developer.paypal.com/webapps/developer/docs/api/#list-plans
+    // at https://developer.paypal.com/docs/api/#list-plans
     $params = array('page_size' => '2');
     $planList = Plan::all($params, $apiContext);
 } catch (Exception $ex) {