Removal of deprecated --sdk-auth flag

[Matt Hansen]

The --sdk-auth flag in az ad sp create-for-rbac command should be removed from

azurechat/docs/4-deploy-to-azure.md

Line 19 in 0d19294

       --name <NAME OF THE CREDENTIAL> --role contributor --scopes /subscriptions/<SUBSCRIPTION ID>/resourceGroups/<RESOURCE GROUP> --sdk-auth --output json 

as it is deprecated and will be removed with reference to Azure/azure-cli#20743.