Correct description of several statistics, including spelling and plurals.

In the stats descriptions, there are some spelling and plural usage
errors.  The term "driver" was also swapped for "client SDK" since we
don't use the term driver anywhere else.

Change-Id: I98de28584dccf4461d75d0534feedb479dd8a628
Reviewed-on: http://review.couchbase.org/34115
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>