use knative-extensions org for istio install (#6509)

Author: dprotaso

docs/versioned/install/yaml-install/serving/install-serving-with-yaml.md

@@ -86,8 +86,8 @@ Follow the procedure for the networking layer of your choice:
     [Advanced Istio installation](../../installing-istio.md) instructions or by running the command:
 
         ```bash
-        kubectl apply -l knative.dev/crd-install=true -f {{ artifact(repo="net-istio",file="istio.yaml")}}
-        kubectl apply -f {{ artifact(repo="net-istio",file="istio.yaml")}}
+        kubectl apply -l knative.dev/crd-install=true -f {{ artifact(repo="net-istio",org="knative-extensions",file="istio.yaml")}}
+        kubectl apply -f {{ artifact(repo="net-istio",org="knative-extensions",file="istio.yaml")}}
         ```
 
     1. Install the Knative Istio controller by running the command: