etc/kamailio.cfg: update version in comments

etc/kamailio.cfg

@@ -1,6 +1,6 @@
 #!KAMAILIO
 #
-# Kamailio SIP Server v5.6 - default configuration script
+# Kamailio SIP Server v5.8 - default configuration script
 #     - web: https://www.kamailio.org
 #     - git: https://github.com/kamailio/kamailio
 #