Fix PHPdoc in the App callback function

src/App.php

@@ -448,7 +448,7 @@ public static function give($implementation)
      * @param string        $method           The method that should be called on the resolved implementation with the
      *                                        specified array arguments.
      *
-     * @return mixed The called method return value.
+     * @return callable The callback function.
      *
      * @throws ContainerException If the id is not a bound implementation or valid class name.
      */