Fixed typo in the yml validation

The `Valid` property should be added to `Acme\HelloBundle\Entity\Author` instead of `Acme\HelloBundle\Author`.
bug #4075 Fixed typo in the yml validation (timothymctim)

This PR was merged into the 2.3 branch.

Discussion
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Fixed typo in the yml validation

The `Valid` property should be added to `Acme\HelloBundle\Entity\Author` instead of `Acme\HelloBundle\Author`.

Commits
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03990ae Fixed typo in the yml validation