Update how-to-backend-mtls-gateway-api.md

added missing command for creating the frontend

Author: PixelRobots

articles/application-gateway/for-containers/how-to-backend-mtls-gateway-api.md

@@ -83,6 +83,7 @@ RESOURCE_NAME='alb-test'
 
 RESOURCE_ID=$(az network alb show --resource-group $RESOURCE_GROUP --name $RESOURCE_NAME --query id -o tsv)
 FRONTEND_NAME='frontend'
+az network alb frontend create -g $RESOURCE_GROUP -n $FRONTEND_NAME --alb-name $AGFC_NAME
 ```
 
 2. Create a Gateway