Make return function in bin/phpss static

Author: luzrain

docs/integrations/symfony.md

@@ -61,7 +61,7 @@ $_SERVER['APP_RUNTIME'] = PHPStreamServerRuntime::class;
 
 require_once \dirname(__DIR__).'/vendor/autoload_runtime.php';
 
-return function (array $context) {
+return static function (array $context) {
     return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
 };
 ```