bug #5608 updated validation.rst (issei-m)

This PR was submitted for the 2.8 branch but it was merged into the 2.3 branch instead (closes #5608).

Discussion
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updated validation.rst

I'm not sure this change is correct but FieldError was renamed to FormError, right?

Commits
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74da3cf updated validation.rst

book/validation.rst

@@ -212,7 +212,7 @@ The ``validator`` service can be used at any time to validate any object.
 In reality, however, you'll usually work with the ``validator`` indirectly
 when working with forms. Symfony's form library uses the ``validator`` service
 internally to validate the underlying object after values have been submitted.
-The constraint violations on the object are converted into ``FieldError``
+The constraint violations on the object are converted into ``FormError``
 objects that can easily be displayed with your form. The typical form submission
 workflow looks like the following from inside a controller::