Fix spelling error in comment

Author: dayallnash

Diff for: samples/RefundOrder.php

@@ -52,9 +52,9 @@ public static function refundOrder($captureId, $debug=false)
 
 /**
  * This is the driver function which invokes the refund capture function with
- * Capture Id to preform refund on capture.
+ * Capture Id to perform refund on capture.
  */
 if (!count(debug_backtrace()))
 {
     RefundOrder::refundOrder('8XL09935J2224701N', true);
-}
+}