add missing response in the example

Author: Frank Gao

specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2020-07-01/examples/AgentPoolsUpgradeNodeImageVersion.json

@@ -7,6 +7,7 @@
     "agentPoolName": "agentpool1"
   },
   "responses": {
+    "200": {},
     "202": {
       "body": {
         "id": "/subscriptions/subid1/resourcegroups/rg1/providers/Microsoft.ContainerService/managedClusters/clustername1/agentPools/agentpool1",