Fix typo

Author: mdoutreluingne

reference/configuration/security.rst

@@ -359,7 +359,7 @@ form_only
 
 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`.
+content type ``application/x-www-form-urlencoded``.
 
 .. versionadded:: 5.4