Unload: update outdated example

Author: arthurvr

entries/unload.xml

@@ -39,7 +39,7 @@ It is not possible to cancel the <code>unload</code> event with <code>.preventDe
     <desc>To display an alert when a page is unloaded:</desc>
     <code><![CDATA[
 $( window ).unload(function() {
-  alert( "Bye now!" );
+  return "Bye now!";
 });
   ]]></code>
   </example>