Update az sp instruction

Author: russdaygh

terraform/readme.md

@@ -1,6 +1,6 @@
 Command to create a new deployment service principal
 ```
-az ad sp create-for-rbac -n DevDeployment --role Contributor --scopes /subscriptions/<insert-subscription-id>
+az ad sp create-for-rbac -n DevDeployment --role Contributor --scopes /subscriptions/<insert-subscription-id>/resourceGroups/<resource-group-name> --sdk-auth
 ```
 Add the JSON output as a secret named `AZURE_CREDENTIALS` to GitHub. I've added this as a secret specific to an environment
 ![alt text](image.png)