minor #11006 Replaces ProtocolClassValidator for the second example (krisdante)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #11006).

Discussion
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Replaces ProtocolClassValidator for the second example

Examples for Class Constraint Validator use ContainsAlphanumeric that was added as property validator. It will not work for a class.
The intention of the second example seems to be the usage of ProtocolClassValidator introduced in this section (chapter).

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Commits
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42007cb Replaces ProtocolClassValidator for the second example

Author: javiereguiluz

validation/custom_constraint.rst

@@ -68,7 +68,7 @@ The validator class is also simple, and only has one required method ``validate(
             if (!$constraint instanceof ContainsAlphanumeric) {
                 throw new UnexpectedTypeException($constraint, ContainsAlphanumeric::class);
             }
-            
+
             // custom constraints should ignore null and empty values to allow
             // other constraints (NotBlank, NotNull, etc.) take care of that
             if (null === $value || '' === $value) {
@@ -214,7 +214,7 @@ not to the property:
     .. code-block:: php-annotations
 
         /**
-         * @AcmeAssert\ContainsAlphanumeric
+         * @AcmeAssert\ProtocolClassValidator
          */
         class AcmeEntity
         {
@@ -226,11 +226,11 @@ not to the property:
         # src/AppBundle/Resources/config/validation.yml
         AppBundle\Entity\AcmeEntity:
             constraints:
-                - AppBundle\Validator\Constraints\ContainsAlphanumeric: ~
+                - AppBundle\Validator\Constraints\ProtocolClassValidator: ~
 
     .. code-block:: xml
 
         <!-- src/AppBundle/Resources/config/validation.xml -->
         <class name="AppBundle\Entity\AcmeEntity">
-            <constraint name="AppBundle\Validator\Constraints\ContainsAlphanumeric" />
+            <constraint name="AppBundle\Validator\Constraints\ProtocolClassValidator" />
         </class>