Fixup typo about replica indexes: they are zero-based

Change-Id: Ibe87f8f03a5e4bd577c1435bb53affcb2789afb5
Reviewed-on: http://review.couchbase.org/27347
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>