Fix variable name in docs

Author: pawel-lewtak

docs/en/reference/custom-integration.rst

@@ -38,7 +38,7 @@ Now place the following code in the ``migrations`` file:
 
     $config = new PhpFile('migrations.php'); // Or use one of the Doctrine\Migrations\Configuration\Configuration\* loaders
 
-    $dependencyFactory = DependencyFactory::fromConnection($config, new ExistingConnection($conn));
+    $dependencyFactory = DependencyFactory::fromConnection($config, new ExistingConnection($connection));
 
     $cli = new Application('Doctrine Migrations');
     $cli->setCatchExceptions(true);