options: remove --key-method from usage message

options: remove --key-method from usage message

Commit 36bef1b52b49ebbc3790635be230e2f30f0532a7 removed
the option but did not delete it from usage text.

Change-Id: I68d3c90c2bdf6f426a9eef81f852fcae2ea47ce9
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230525144657.40732-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26726.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e8a026ac770592670b0dcf8f81cee6a98b4b4f65)