Fix Exception message that leads in the wrong direction.

An exception is raised when the number of vBuckets is not correct,
but the message states "buckets" instead of "vBuckets", which may
lead to confusion.

Change-Id: I2be6fd1ddcc38837dfc86196d324d6940eb16e99
Reviewed-on: http://review.couchbase.org/25661
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>