Fix one help description of rbd driver
Fix one help description of rbd driver

One symbol '-' of report_dynamic_total_capacity was removed
by commit https://review.openstack.org/#/c/564068/. In fact,
The last two symbols '-' are a pair of.
Maybe the symbol '-' makes person confused, so that I replace
'-' with '(' and ')'.

Change-Id: I7da0f4715ba64e1384f7bf345ac850ca819555e7

mahak/cinder,openstack/cinder,mahak/cinder,openstack/cinder