Updated examples/README and parser for run_summarization_finetuning
1. Updated `examples/README.md` to change default `--model_type` and `--model_name_or_path` to `bert` and `bert_base_cased` because `bert2bert` just won't work
2. Updated `examples/run_summarization_finetuning.py` parser to take in `--do-train` instead of `--do-train=True` for consistency with other examples and `--model_type` + `--model_name_or_path`
3. Changed `add_special_tokens_single_sequence` to `build_inputs_with_special_tokens` in `examples/utils_summarization.py`  
Let's wait that the summarization script is finalized before merging this.