sss_ssh_knownhostsproxy: fix typo pubkey -> pubkeys

In commit 36f2fe8 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 Fidêncio <fidencio@redhat.com>

src/sss_client/ssh/sss_ssh_knownhostsproxy.c

@@ -206,7 +206,7 @@ int main(int argc, const char **argv)
           _("The port to use to connect to the host"), NULL },
         { "domain", 'd', POPT_ARG_STRING, &pc_domain, 0,
           _("The SSSD domain to use"), NULL },
-        { "pubkey", 'k', POPT_ARG_NONE, &pc_pubkeys, 0,
+        { "pubkeys", 'k', POPT_ARG_NONE, &pc_pubkeys, 0,
           _("Print the host ssh public keys"), NULL },
         POPT_TABLEEND
     };