Update systemd service name param to match command

Update systemd service name param to match command

The service name displays `%I` which invokes systemd's path mangling
(notably, converting `-` to `/`), suggesting you need to provide an
encoded parameter (via e.g. `systemd-escape`), but the start command
itself uses `%i` which doesn't do the conversion.

This updates the service name to match the start command.

Signed-off-by: Terrance <git@terrance.allofti.me>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20250729180014.11550-2-sourceforge@terrance.allofti.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32423.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>