minor symfony#8780 Update without_class.rst (tabbi89)

This PR was merged into the 2.7 branch.

Discussion
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Update without_class.rst

`$form->isValid()` no validation is done here this method only returns bool flag if any errors occurred during validation or false if a form was not submitted. Validation happens when executing `handleRequest()` or `submit()`.

Commits
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b4c203e Update without_class.rst

Author: xabbuh

form/without_class.rst

@@ -63,7 +63,7 @@ an array.
 Adding Validation
 ~~~~~~~~~~~~~~~~~
 
-The only missing piece is validation. Usually, when you call ``$form->isValid()``,
+The only missing piece is validation. Usually, when you call ``$form->handleRequest($request)``,
 the object is validated by reading the constraints that you applied to that
 class. If your form is mapped to an object (i.e. you're using the ``data_class``
 option or passing an object to your form), this is almost always the approach