minor #8092 Update custom_constraint.rst (DonCallisto)

This PR was submitted for the 3.3 branch but it was merged into the 2.7 branch instead (closes #8092).

Discussion
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Update custom_constraint.rst

Updated custom_constraint.rst with Validator::class syntax

Commits
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4d5b4fc Update custom_constraint.rst

Author: xabbuh

validation/custom_constraint.rst

@@ -45,7 +45,7 @@ includes some simple default logic::
     // in the base Symfony\Component\Validator\Constraint class
     public function validatedBy()
     {
-        return get_class($this).'Validator';
+        return ContainsAlphanumericValidator::class;
     }
 
 In other words, if you create a custom ``Constraint`` (e.g. ``MyConstraint``),