Update 10_Cluster_health.md

最后一句在
https://github.com/elasticsearch/elasticsearch-definitive-guide/blob/master/020_Distributed_Cluster/10_Cluster_health.asciidoc
的原文是：
In the rest of this chapter we explain what primary and replica shards are and explain the practical implications of each of the above colors.
个人认为翻译时注释都用单数即可。