Update the replay-window backtrack log message

Update the replay-window backtrack log message

The man pages reference a logging message which has been rephrased
in ac1310528a248c99e039e7afaf48724ad1b7f10e. This commit updates the
man page message to reflect the change for improved grep-ability.

Signed-off-by: Martin Janů <martin.janu@protonmail.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <SVrvuTydxR6Qs_mvwvG7mqT8iLV0inlcCMXoenZTMI8M0LkosV4pZsH9m_XCTwcRWAPN5H8Zdro0ubhJrnSp6v5KC2ZNAL9So0Y2SKiSe7g=@protonmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24472.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 98cbd2914cb728bfaa6ce36a968e944555c57d53)