Command-line interface Document Bug
There is a bug in README.md about Command-line interface:
`export BERT_BASE_DIR=chinese_L-12_H-768_A-12`

**Wrong:**
```
pytorch_pretrained_bert convert_tf_checkpoint_to_pytorch \
   --tf_checkpoint_path $BERT_BASE_DIR/bert_model.ckpt.index \
   --bert_config_file $BERT_BASE_DIR/bert_config.json \
   --pytorch_dump_path $BERT_BASE_DIR/pytorch_model.bin
```

**Right:**
```
pytorch_pretrained_bert convert_tf_checkpoint_to_pytorch \
  $BERT_BASE_DIR/bert_model.ckpt.index \
  $BERT_BASE_DIR/bert_config.json \
  $BERT_BASE_DIR/pytorch_model.bin
```

Thanks!