man/sss_ssh_knownhostsproxy: fix typo pubkeys -> pubkey
In commit 36f2fe8f63 a discrepancy between the command line option and
the manpage has been introduced.
Related:
https://pagure.io/SSSD/sssd/issue/3542
Signed-off-by: Fabiano FidĂȘncio <fidencio@redhat.com>
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>