fix(laravel): undefined variable app

Author: soyuka

src/Laravel/ApiPlatformProvider.php

@@ -1092,7 +1092,7 @@ private function registerGraphQl(): void
             );
         });
 
-        $app->singleton('api_platform.graphql.runtime_operation_metadata_factory', function (Application $app) {
+        $this->app->singleton('api_platform.graphql.runtime_operation_metadata_factory', function (Application $app) {
             return new RuntimeOperationMetadataFactory(
                 $app->make(ResourceMetadataCollectionFactoryInterface::class),
                 $app->make(UrlGeneratorRouter::class)