man.8: Add bookmark to list of types

While checking bash_completion I missed bookmark as type.

# zfs get type zpool2#b
NAME      PROPERTY  VALUE     SOURCE
zpool2#b  type      bookmark  -

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Grischa Zengel <github.zfsonlinux@zengel.info>
Closes #10419