Update README.md

Fix for

```> > > > 04/09/2019 21:39:38 - INFO - __main__ -   device: cuda n_gpu: 1, distributed training: False, 16-bits training: False
Traceback (most recent call last):
  File "/home/ubuntu/pytorch-pretrained-BERT/examples/lm_finetuning/simple_lm_finetuning.py", line 642, in <module>
    main()
  File "/home/ubuntu/pytorch-pretrained-BERT/examples/lm_finetuning/simple_lm_finetuning.py", line 502, in main
    raise ValueError("Training is currently the only implemented execution option. Please set `do_train`.")
ValueError: Training is currently the only implemented execution option. Please set `do_train`.
```

examples/lm_finetuning/README.md

@@ -37,6 +37,7 @@ python3 simple_lm_finetuning.py
 --bert_model bert-base-uncased 
 --do_lower_case 
 --output_dir finetuned_lm/
+--do_train
 ```
 
 ### Pregenerating training data
@@ -60,4 +61,4 @@ python3 finetune_on_pregenerated.py
 --do_lower_case
 --output_dir finetuned_lm/
 --epochs 3
-```
+```
\ No newline at end of file