Fix defaults for LCB_DURABILITY_INTERVAL in man

Change-Id: If483f18f9762ef681616a0b77ae1236a86a2049f
Reviewed-on: http://review.couchbase.org/28331
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>