man/systemd-cryptenroll: Fix sd-boot manvolnum from 8 to 7
Off-by-one error in the documentation index. The volume number for
systemd-boot/sd-boot is 7.
Signed-off-by: Morten Linderud <morten@linderud.pw>
(cherry picked from commit 29d898cc7be4b2e91f13bba83fef46bbe995f08e)