minor #11431 config/validator/validation.yaml fix (pandalowry)

This PR was submitted for the master branch but it was merged into the 4.2 branch instead (closes #11431).

Discussion
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config/validator/validation.yaml fix

Validations in yaml will not work for a student passing through this section, since # config/validation.yaml was set while in the documentation for the component https://symfony.com/doc/current/validation.html#the-basics- Of-validation clearly states that: "These rules are usually defined using PHP code or annotations but they can also be defined as a validation.yaml or validation.xml file inside the config/validator/ directory:"

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Commits
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ab09fbb config/validator/validation.yaml fix

Author: javiereguiluz

forms.rst

@@ -336,7 +336,7 @@ object.
 
     .. code-block:: yaml
 
-        # config/validation.yaml
+        # config/validator/validation.yaml
         App\Entity\Task:
             properties:
                 task: