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
(cherry picked from commit 1df9b26b51ff3a153b93d79e25bef454e08a8e38)
Merge "Fix docstring for disk_cachemodes" into stable/havana