Incorrect RFC reference

From manual page: https://php.net/class.datetimeinterface

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In the [DateTimeInterface::ATOM](https://www.php.net/manual/en/class.datetimeinterface.php#datetimeinterface.constants.atom) [string](https://www.php.net/manual/en/language.types.string.php)
[DATE_ATOM](https://www.php.net/manual/en/datetime.constants.php#constant.date-atom) section `RFC 3399` is referenced, it should be `RFC 3339`