[Heartbeat] Fix example in docs for autodiscover (elastic#11891)

The docs used `hosts` instead of the correct `urls` config key.

Fixes elastic#11643

heartbeat/docs/autodiscover-kubernetes-config.asciidoc

@@ -12,7 +12,7 @@ heartbeat.autodiscover:
               kubernetes.annotations.prometheus.io.scrape: "true"
           config:
             - type: http
-              hosts: ["${data.host}:${data.port}"]
+              urls: ["${data.host}:${data.port}"]
               schedule: "@every 1s"
               timeout: 1s
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