fixed ns_heart crash caused by wrong format string

Change-Id: I1769d7397dad7ce9f72f53f2a9dfb25a6df413e6
Reviewed-on: http://review.couchbase.org/30352
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>