Remove ')' in stats name for backfill_mem_items

Change-Id: Iefcc95f4a7d705db1db4f7eb1e97e9a79db3ac24
Reviewed-on: http://review.couchbase.org/44435
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>