Update PaymentMethod.php

Update the docstring to return `void`, since that is what the `deletePaymentMethod` function actually returns

src/PaymentMethod.php

@@ -51,7 +51,7 @@ public function __construct($owner, StripePaymentMethod $paymentMethod)
     /**
      * Delete the payment method.
      *
-     * @return \Stripe\PaymentMethod
+     * @return void
      */
     public function delete()
     {