Call to Sprintf malformed

See: http://play.golang.org/p/OoZfNXBmHA

For just indices, a single '%s' suffices. I think the second '%s' may have been intended for params (http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/cluster-health.html), but adding those as an argument to the Health function would be a breaking API change. Let me know if you would like that change made to the API.