tutorials/mini-howto-admin/main.md: updated relative links

tutorials/mini-howto-admin/main.md

@@ -3,7 +3,5 @@
 A collection of mini how-to tutorials addressing specific use cases of
 Kamailio.
 
--   [User Authentication Using LDAP
-    Backend](/tutorials/mini-howto-admin/ldap-user-auth)
--   [Call Forwarding Using
-    AVP](/tutorials/mini-howto-admin/call_forwarding)
+-   [User Authentication Using LDAP Backend](tutorials/mini-howto-admin/ldap-user-auth.md)
+-   [Call Forwarding Using AVP](tutorials/mini-howto-admin/call_forwarding.md)