minor #7046 Fix incorrect callback validation example (mvar)

This PR was merged into the 2.7 branch.

Discussion
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Fix incorrect callback validation example

Current example in documentation leads into an exception:

> Uncaught PHP Exception Symfony\Component\Validator\Exception\ConstraintDefinitionException: "["validate"] targeted by Callback constraint is not a valid callable"

Commits
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9674500 Fix incorrect callback validation example

Author: xabbuh

reference/constraints/Callback.rst

@@ -59,7 +59,7 @@ Configuration
         # src/AppBundle/Resources/config/validation.yml
         AppBundle\Entity\Author:
             constraints:
-                - Callback: [validate]
+                - Callback: validate
 
     .. code-block:: xml