Fix docblock type

src/Silex/Application.php

@@ -413,7 +413,7 @@ class Application extends \Pimple implements HttpKernelInterface, TerminableInte
      * an instance of Response. When this occurs, all suitable handlers will be 
      * called, until one returns a Response object.  
      *
-     * @param mixed   $callback View handler callback
+     * @param callable   $callback View handler callback
      * @param integer $priority The higher this value, the earlier an event
      *                          listener will be triggered in the chain (defaults to 0)
      */