plugin/health: update README (#1739)

* plugin/health: update README

Make more clear in the readme that health is limited to 1 server.

Fixes #1722

* rephrase and remove ~~~ corefile because it will fail

plugin/health/README.md

@@ -48,6 +48,17 @@ net {
 }
 ~~~
 
+Note that if you format this in one server block you will get an error on startup, that the second
+server can't setup the health plugin (on the same port).
+
+~~~ txt
+com net {
+    whoami
+    erratic
+    health :8080
+}
+~~~~
+
 ## Plugins
 
 Any plugin that implements the Healther interface will be used to report health.