Fix ZPOOL_VDEV_NAME_PATH option description

The corresponding zpool status option is -P and not -p. Update
this description to reference the correct option.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9803