ipsec: add missing statements in example

By default the strongswan client will attempt to authenticate the
connection using PKI, which isn't going to work if the other side
is doing PSK...

howto/ipsec.md

@@ -162,6 +162,9 @@ then on "curie", the following configuration was added to
 
       auto=route
 
+      authby=secret
+      keyexchange=ikev2
+
 The secret generated on `chi-node-01` for the roaming client (in
 `/etc/ipsec.secrets.d/20-curie.anarc.at.secrets` was copied over to
 the roaming client, in `/etc/ipsec.secrets` (by default, apparmor