Fixed typo about _token field name for CSRF protection

Author: JMLamodiere

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::