Fix XML config example

Author: javiereguiluz

security/csrf.rst

@@ -113,7 +113,7 @@ Globally, you can configure it under the ``framework.form`` option:
 
             <framework:config>
                 <framework:form>
-                    <framework:csrf-protection enabled="true" field_name="custom_token_name"/>
+                    <framework:csrf-protection enabled="true" field-name="custom_token_name"/>
                 </framework:form>
             </framework:config>
         </container>