bug #5356 [Form] Fixed typo about _token field name for CSRF protection (JMLamodiere)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Fixed typo about _token field name for CSRF protection

As seen [here](https://github.com/symfony/symfony/blob/2.3/src/Symfony/Component/Form/Extension/Csrf/Type/FormTypeCsrfExtension.php#L53)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

Commits
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c4bc7e7 Fixed typo about _token field name for CSRF protection

Author: wouterj

components/form/introduction.rst

@@ -138,7 +138,7 @@ above snippet and make sure that nobody except your web server can access
 the secret.
 
 Internally, this extension will automatically add a hidden field to every
-form (called ``__token`` by default) whose value is automatically generated
+form (called ``_token`` by default) whose value is automatically generated
 and validated when binding the form.
 
 .. tip::