Pass folder names for the foldersort function

Bring the description in the template offlineimap.conf in sync to the
actual implementation: pass folder names to the sorting function, not
the offlineimap.folder.IMAP.IMAPFolder objects themselves.

GitHub issue: https://github.com/OfflineIMAP/offlineimap/issues/27
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>