fix retry example in virtual service (#3514)

Signed-off-by: Rama Chavali <rama.rao@salesforce.com>

Author: ramaraochavali

networking/v1alpha3/virtual_service.pb.go

@@ -450,7 +450,7 @@ message HTTPRoute {
   //
   // ```yaml
   // attempts: 2
-  // retryOn: "connect-failure,refused-stream,unavailable,cancelled,503"
+  // retryOn: "connect-failure,refused-stream,unavailable,cancelled"
   // ```
   //
   // This can be customized in [`Mesh Config` `defaultHttpRetryPolicy`](https://istio.io/latest/docs/reference/config/istio.mesh.v1alpha1/#MeshConfig).