fix: the readme and train codes do not match

[LongQIByte]

Download [BERT-Base-Cased](https://huggingface.co/bert-base-cased/tree/main) which contains `pytroch_model.bin`, `vocab.txt` and `config.json`. Put these under `./pretrain_models/`.

👆🏻👆🏻👆🏻Above will display an error, do not have this folder(/pretrain_models/bert_base_cased/)

# in train.py

bert_config = BertConfig.from_json_file(os.path.join(params.bert_model_dir, 'bert_config.json'))

👆🏻👆🏻👆🏻Above will display an error, can't find bert_config.json.In fact just download should be config.json