Correct PyTorch distributed training command in examples/README.md
Running the command currently detailed in the documentation yields 
`58.4/70.3 EM/F1` with Ubuntu 18.04 and torch 1.4.0, not `86.9/93.1` as promised. It also looks wrong because we're using a cased model with `--do_lower_case`. 

Switching it to match the PyTorch distributed training example given in the main README gives me the approximately-correct `86.68/93.03` results.
That's cool, thanks @jarednielsen !