MDL-61741 enrol_paypal: Fix the IPN verification endpoint URL

IPN protocol spec has different URL for sending the verification
request. Better to have it fixed even if the existing value still
seems to work - to avoid unexpected surprise in the future.

Source: https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNImplementation/#specs

enrol/paypal/ipn.php

@@ -103,7 +103,7 @@
 $plugin = enrol_get_plugin('paypal');
 
 /// Open a connection back to PayPal to validate the data
-$paypaladdr = empty($CFG->usepaypalsandbox) ? 'www.paypal.com' : 'www.sandbox.paypal.com';
+$paypaladdr = empty($CFG->usepaypalsandbox) ? 'ipnpb.paypal.com' : 'ipnpb.sandbox.paypal.com';
 $c = new curl();
 $options = array(
     'returntransfer' => true,