Updated readme.

README.md

@@ -535,8 +535,9 @@ The default behavior is to send a generic `HTTP 500 Internal Server Error` respo
 You can override this behavior for your own needs:
 
 ```php
-Flight::map('error', function(){
+Flight::map('error', function(Exception $ex){
     // Handle error
+    echo $ex->getTraceAsString();
 });
 ```