Fix the typo in volume read-only access cli command

The command to set a volume to read-only access has a typo
in the user guide. The command listed in the guide is
cinder read-only-mode-update VOLUME BOOLEAN
but the correct command is
cinder readonly-mode-update VOLUME BOOLEAN

Change-Id: I60eae0fd67faa153b92def69dd5b567738e9dcbb

Closes-Bug: #1328282

Change-Id: I60eae0fd67faa153b92def69dd5b567738e9dcbb
Merge "Fix the typo in volume read-only access cli command"