Should be ``additional_formats``.

Same typo at:
https://github.com/symfony/symfony/pull/8944/files#L1R363
https://github.com/symfony/symfony/pull/8944/files#L2R19
https://github.com/symfony/symfony/pull/8944/files#L3R14
https://github.com/symfony/symfony/pull/8944/files#L4R52