doc: Fix hyperlinks in openvpn(8)

doc: Fix hyperlinks in openvpn(8)

* http://www.cs.ucsd.edu/users/mihir/papers/hmac.html - 404, RFC104 basically
* http://sites.inka.de/sites/bigred/devel/tcp-tcp.html - 404, unfortunately
* http://www.ietf.org/rfc/rfc2246.txt - HTTPS upgrade to the HTML version

Signed-off-by: Christian Kujau <github@nerdbynature.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20251006144249.23672-2-lists@nerdbynature.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59242864/
Signed-off-by: Gert Doering <gert@greenie.muc.de>