T7635: Show OpenConnect certificate based authentication mode option in configuration example.

Author: giga1699

docs/configuration/vpn/openconnect.rst

@@ -47,7 +47,7 @@ Server Configuration
 .. code-block:: none
 
   set vpn openconnect authentication local-users username <user> password <pass>
-  set vpn openconnect authentication mode <local password|radius>
+  set vpn openconnect authentication mode <local password|radius|cert>
   set vpn openconnect network-settings client-ip-settings subnet <subnet>
   set vpn openconnect network-settings name-server <address>
   set vpn openconnect network-settings name-server <address>