Fix docstring for disk_cachemodes

The docstring for the disk_cachemodes option implied a syntax that is
incorrect for a ListOpt.  Change the example to match exactly what you
would put in the config file for it to work.

Change-Id: Ie999d2799dba6c47facdc771e1cf6081d28d6b85
Closes-Bug: #1106423
Merge "Fix docstring for disk_cachemodes"