don't show deprecated usage of Yaml::parse()

Author: xabbuh

components/class_loader/psr4_class_loader.rst

@@ -55,7 +55,7 @@ first need to configure the ``Psr4ClassLoader``:
     $loader->addPrefix('Symfony\\Component\\Yaml\\', __DIR__.'/lib/Yaml');
     $loader->register();
 
-    $data = Yaml::parse(__DIR__.'/config.yml');
+    $data = Yaml::parse(file_get_contents(__DIR__.'/config.yml'));
 
 First of all, the class loader is loaded manually using a ``require``
 statement, since there is no autoload mechanism yet. With the