Add missing $

This avoids this error message:

"The term 'groupId=$(az group show --name <my-name> --query id --output tsv)' is not recognized as a name of a cmdlet, function, script file, or executable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again."

Author: klacol

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

@@ -51,7 +51,7 @@ In the GitHub workflow, you need to supply Azure credentials to authenticate to
 First, get the resource ID of your resource group. Substitute the name of your group in the following [az group show][az-group-show] command:
 
 ```azurecli
-groupId=$(az group show \
+$groupId=$(az group show \
   --name <resource-group-name> \
   --query id --output tsv)
 ```