minor #10457 [Form][Validation] User $user instead of $users (lyrixx)

This PR was merged into the 2.8 branch.

Discussion
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[Form][Validation] User $user instead of $users

Fixed example where it feels more natural to register only one user and not a collection of users

Commits
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d1beaff [Form][Validation] User $user instead of $users

Author: javiereguiluz

form/validation_groups.rst

@@ -10,7 +10,7 @@ Validation Groups
 If your object takes advantage of :doc:`validation groups </validation/groups>`,
 you'll need to specify which validation group(s) your form should use::
 
-    $form = $this->createFormBuilder($users, array(
+    $form = $this->createFormBuilder($user, array(
         'validation_groups' => array('registration'),
     ))->add(...);