Update service name for istio gateway

When installing knative with GitLab managed apps. There is no service
named `knative-ingressgateway` under the `istio-system` namespace.

For example:

<details>
<summary> kubectl get svc --namespace=isito-system </summary>

```
cluster-local-gateway ClusterIP
istio-ingressgateway  LoadBalancer
istio-pilot           ClusterIP
istio-telemetry       ClusterIP
```

</details>

doc/user/clusters/applications.md

@@ -212,7 +212,7 @@ kubectl get service --namespace=gitlab-managed-apps ingress-nginx-ingress-contro
 For Istio/Knative, the command will be different:
 
 ```shell
-kubectl get svc --namespace=istio-system knative-ingressgateway -o jsonpath='{.status.loadBalancer.ingress[0].ip} '
+kubectl get svc --namespace=istio-system istio-ingressgateway -o jsonpath='{.status.loadBalancer.ingress[0].ip} '
 ```
 
 Otherwise, you can list the IP addresses of all load balancers: