[AttributeBundle] Fix key names for validation_groups config

Q | A
---|---
Bug fix? | yes
New feature? | no
BC breaks? | no
Deprecations? | no
Fixed tickets | -

Thanks @pjedrzejewski for confirming this issue.

Fixes issue where [conditions to check if validation groups for attribute and attribute value were configured][1] would always be false, and never used.

[1]: https://github.com/Sylius/Sylius/blob/master/src/Sylius/Bundle/AttributeBundle/DependencyInjection/SyliusAttributeExtension.php#L56