Fix a typo in health plugin readme
## Description
By enabling *health* any plugin that implements
-[healt.Healther interface](https://godoc.org/github.com/coredns/coredns/plugin/health#Healther)
+[health.Healther interface](https://godoc.org/github.com/coredns/coredns/plugin/health#Healther)
will be queried for it's health. The combined health is exported, by default, on port 8080/health .
## Syntax
