Fix cinder option to set a volume to read-only access

Under the "Manage volume" section of the OpenStack End User Guide,
it says to use "cinder read-only-mode-update VOLUME BOOLEAN" to
set the volume to read-only. The option "read-only-mode-update" is
incorrect. It should be "readonly-mode-update". This patch corrects
the option used.

Change-Id: I863272f98a3289c6bab877a618c8f6e898800bd3
Closes-Bug: #1311283
Merge "Fix cinder option to set a volume to read-only access"