Fix imports in health sample documentation

Fixes gh-6312

Author: philwebb

spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -326,6 +326,7 @@ additional details to be displayed.
 
 [source,java,indent=0]
 ----
+	import org.springframework.boot.actuate.health.Health;
 	import org.springframework.boot.actuate.health.HealthIndicator;
 	import org.springframework.stereotype.Component;