zstreamdump: -d option is not documented in manpage

This change simply documents the missing -d (dump contents) option in
zstreamdump(8).

Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8369