HAProxy example: Improved formating

It was not obvious that it must be two lines.

Signed-off-by: Fonata <Fonata@users.noreply.github.com>

admin_manual/configuration_server/reverse_proxy_configuration.rst

@@ -75,6 +75,7 @@ Thanks to `@pauvos <https://github.com/pauvos>`_ for traefik example.
 
 HAProxy
 ^^^^^^^
+::
 
   acl url_discovery path /.well-known/caldav /.well-known/carddav
   http-request redirect location /remote.php/dav/ code 301 if url_discovery