missing resource-group-name in az acr show

The resource group of the acr needs to be provided, or a default resource group would be attempted.
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cc: @tomvcassidy

Author: yodobrin

articles/container-instances/container-instances-github-action.md

@@ -97,6 +97,7 @@ Get the resource ID of your container registry. Substitute the name of your regi
 ```azurecli
 registryId=$(az acr show \
   --name <registry-name> \
+  --resource-group <resource-group-name> \
   --query id --output tsv)
 ```