minor symfony#8435 Fix explanation when Creating a Validator (lemonlab)

This PR was submitted for the 3.3 branch but it was merged into the 2.7 branch instead (closes symfony#8435).

Discussion
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Fix explanation when Creating a Validator

In the base class Symfony\Component\Validator\Constraint, the method validatedBy() contain this code instead.
This makes the explanation understandable.

Commits
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2e61702 Fix explanation when Creating a Validator

Author: wouterj

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 ContainsAlphanumericValidator::class;
+        return get_class($this).'Validator';
     }
 
 In other words, if you create a custom ``Constraint`` (e.g. ``MyConstraint``),