[docs] Correct form component version in docs

Validator ~2.1 pulls in a new version of the validator component where `Symfony\Component\Validator\Mapping\ClassMetaDataFactory` has `getMetaDataFor` instead of `getClassMetaData` - this causes an exception from `Symfony\Component\Form\Extension\Validator`, line 39 - where the 2.1 version pulled in by `~2.1.4` calls getClassMetaData. 

This change has fixed it for me - just depends whether there was a reason for holding the form component at 2.1 ?