Fix typo

Author: mdoutreluingne

reference/configuration/security.rst

@@ -357,7 +357,7 @@ form_only
 
 **type**: ``boolean`` **default**: ``false``
 
-By setting this option to ``true`, a content type check will be performed when the login form is submitted
+By setting this option to ``true``, a content type check will be performed when the login form is submitted
 (i.e. the login form will be processed if it is the form data, so with a
 content type `application/x-www-form-urlencoded`.