minor #10801 Added use statement for UnexpectedValueException (skipton-io)

This PR was merged into the 4.2 branch.

Discussion
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Added use statement for UnexpectedValueException

use Symfony\Component\Validator\Exception\UnexpectedValueException;

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Commits
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d98467e Added use statement for UnexpectedValueException

Author: javiereguiluz

validation/custom_constraint.rst

@@ -60,6 +60,7 @@ The validator class is also simple, and only has one required method ``validate(
     use Symfony\Component\Validator\Constraint;
     use Symfony\Component\Validator\ConstraintValidator;
     use Symfony\Component\Validator\Exception\UnexpectedTypeException;
+    use Symfony\Component\Validator\Exception\UnexpectedValueException;
 
     class ContainsAlphanumericValidator extends ConstraintValidator
     {