docs: Update k8s startup port to be on same as liveness port (GoogleCloudPlatform#2519)

Fixes a typo in the README.md for the kubernetes example. 

Fixes GoogleCloudPlatform#2473

Author: Ryuheeeei

examples/k8s-health-check/README.md

@@ -43,7 +43,7 @@ startupProbe:
    # the pod only after the proxy has successfully started.
    httpGet:
       path: /startup
-      port: 9090
+      port: 9801
    periodSeconds: 1
    timeoutSeconds: 5
    failureThreshold: 20