tls: docs - fix example for setting xavp_cfg

(cherry picked from commit 4f42115ce29d446ad60b32af9ad370d9b2d847c2)

src/modules/tls/doc/params.xml

@@ -1208,7 +1208,7 @@ modparam("tls", "config", "/usr/local/etc/kamailio/tls.cfg")
   modparam("tls", "xavp_cfg", "tls")
  ...
   $xavp(tls=>server_name) = "kamailio.org";
-  $xavp(tls=>server_id) = "kamailio.org";
+  $xavp(tls[0]=>server_id) = "kamailio.org";
   $du = "sip:kamailio.org:5061;transport=tls";
   route(RELAY);
 ...