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。
Merge pull request #3 from iridiumcao/patch-3

Update 10_Cluster_health.md