Add missing link

Author: OskarStark

notifier.rst

@@ -1002,6 +1002,7 @@ is dispatched. Listeners receive a
 .. _`Slack`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/Slack/README.md
 .. _`Sms77`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/Sms77/README.md
 .. _`SmsBiuras`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/SmsBiuras/README.md
+.. _`Smsbox`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/Smsbox/README.md
 .. _`Smsapi`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/Smsapi/README.md
 .. _`Smsc`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/Smsc/README.md
 .. _`SpotHit`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Notifier/Bridge/SpotHit/README.md