A little mistakes in tony-example README

[daugraph]

I think there is a little mistakes in tony-example, as mentioned in #492, because mnist_distributed.py is write in tensorflow 1.x, but in README, it said

Build a Python virtual environment with TensorFlow 2.3 installed

user should install tensorflow 1.x to run mnist_distributed.py

[zuston]

Nice catch. Looks that tf2.x is used in mnist_keras_distributed.py, not mnist_distributed.py

I will fix it later. Thanks