Update README.md (#225)

If you set both --size and --size-low-pri you receive an error...

dciborow@6433739-0524:/mnt/c/GIT/ABN/spark-recommender$ aztk spark cluster create --id testCluster1 --size 0 --size-low-pri 2 --vm-size standard_d2_v2
usage: aztk spark cluster create [-h] [--id CLUSTER_ID]
                                 [--size SIZE | --size-low-pri SIZE_LOW_PRI]
                                 [--vm-size VM_SIZE] [--username USERNAME]
                                 [--password PASSWORD] [--ssh-key SSH_KEY]
                                 [--docker-repo DOCKER_REPO] [--no-wait]
                                 [--wait]
aztk spark cluster create: error: argument --size-low-pri: not allowed with argument --size

README.md

@@ -71,11 +71,10 @@ aztk spark cluster create \
 ```
 You can find more information on VM sizes [here.](https://docs.microsoft.com/en-us/azure/virtual-machines/linux/sizes) Please note that you must use the official SKU name when setting your VM size - they usually come in the form: "standard_d2_v2".
 
-You can also create your cluster with [low-priority](https://docs.microsoft.com/en-us/azure/batch/batch-low-pri-vms) VMs at an 80% discount by using `--size-low-pri` instead of `--size` (we have to set `--size 0` as we currently do not support mixed low-priority and dedicated VMs):
+You can also create your cluster with [low-priority](https://docs.microsoft.com/en-us/azure/batch/batch-low-pri-vms) VMs at an 80% discount by using `--size-low-pri` instead of `--size` (we currently do not support mixed low-priority and dedicated VMs):
 ```
 aztk spark cluster create \
     --id <my_cluster_id> \
-    --size 0 \
     --size-low-pri <number_of_low-pri_nodes> \
     --vm-size <vm_size>
 ```