Add This field is missing RU translation

This PR was submitted on the symfony/Validator read-only repository and moved automatically to the main Symfony repository (closes symfony/Validator#4).

<source>This field is missing.</source>
Have no translation. Not exists in EN version too. Just forgot? 
Symfony\Component\Validator\Constraints\Collection()
public $missingFieldsMessage = 'This field is missing.';