Fix two doc issues (#3278)

* Correct istio yaml file

* Mixer destination rules are expected to exist

content/docs/tasks/security/authn-policy/index.md

@@ -15,7 +15,7 @@ the underlying concepts in the [authentication overview](/docs/concepts/security
 * Understand Istio [authentication policy](/docs/concepts/security/#authentication-policies) and related
 [mutual TLS authentication](/docs/concepts/security/#mutual-tls-authentication) concepts.
 
-* Have a Kubernetes cluster with Istio installed, without global mutual TLS enabled (e.g use `install/kubernetes/istio.yaml` as described in
+* Have a Kubernetes cluster with Istio installed, without global mutual TLS enabled (e.g use `install/kubernetes/istio-demo.yaml` as described in
 [installation steps](/docs/setup/kubernetes/quick-start/#installation-steps), or set `global.mtls.enabled` to false using
 [Helm](/docs/setup/kubernetes/helm-install/)).
 

content/docs/tasks/security/mtls-migration/index.md

@@ -58,13 +58,18 @@ down once the migration is done.
     sleep.legacy to httpbin.foo: 200
     {{< /text >}}
 
-* Also verify that there are no authentication policy or destination rule in the system
+* Also verify that there are no authentication policy or destination rules (except mixer's) in the system:
 
     {{< text bash >}}
     $ kubectl get policies.authentication.istio.io --all-namespaces
     No resources found.
+    {{< /text >}}
+
+    {{< text bash >}}
     $ kubectl get destinationrule --all-namespaces
-    No resources found.
+    NAMESPACE      NAME              AGE
+    istio-system   istio-policy      25m
+    istio-system   istio-telemetry   25m
     {{< /text >}}
 
 ## Configure the server to accept both mutual TLS and plain text traffic