1-start-with-mnist missing environment variable

[jackhuang007]

in section "5.通过Arena提交训练任务", "%env USER_DATA_NAME=training-data" is required but missing

# Set the Job Name
%env JOB_NAME=tf-mnist

%env USER_DATA_NAME=training-data
# Submit a training job 
# using code and data from Data Volume
!arena submit tf \
             --name=$JOB_NAME \
             --gpus=1 \
             --data=$USER_DATA_NAME:/training \
             --tensorboard \
             --image=tensorflow/tensorflow:1.11.0-gpu-py3 \
             --logdir=/training/output/mnist \
             "python /training/models/tensorflow-sample-code/tfjob/docker/mnist/main.py --max_steps 5000 --data_dir /training/dataset/mnist --log_dir /training/output/mnist"