Add zone argument for xpk commands in multihost RL tutorials

Author: gobbleturk

docs/tutorials/posttraining/rl_on_multi_host.md

@@ -83,6 +83,7 @@ export MAXTEXT_CKPT_PATH=${BASE_OUTPUT_DIRECTORY?}/${WORKLOAD?}/0/items
 export TPU_TYPE=<TPU Type> # e.g., 'v5p-128'
 export TPU_CLUSTER=<cluster name>
 export PROJECT_ID=<GCP project ID>
+export ZONE=<GCP zone>
 export CLOUD_IMAGE_NAME=<your artifact registry image> # Name for the Docker image to be built
 ```