add missing backslash

Author: fritzmg

docs/dev/framework/hooks.md

@@ -66,7 +66,7 @@ You can add your custom logic to hooks by extending the `TL_HOOKS` key in the
 
 ```php
 // contao/config.php
-$GLOBALS['TL_HOOKS']['activateAccount'][] = [App\EventListener\ActivateAccountListener::class, 'onActivateAccount'];
+$GLOBALS['TL_HOOKS']['activateAccount'][] = [\App\EventListener\ActivateAccountListener::class, 'onActivateAccount'];
 ```
 
 In this case, the method `onActivateAccount` in the class `App\EventListener\ActivateAccountListener` is called as soon as the hook