typo in documentation

Change-Id: I7ade304c737cb999667d754ff227c8ecc53f7927
Reviewed-on: http://review.couchbase.org/37996
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>