doc: minor fix in ceph-authtool man page

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@gmail.com>
Merge pull request #3019 from theanalyst/doc/typo/ceph-auth

doc: minor fix in ceph-authtool man page

Reviewed-by: Loic Dachary <ldachary@redhat.com>