fix doctest failure

  >>> from transformers import BertTokenizerFast

  >>> tokenizer = BertTokenizerFast.from_pretrained("Rostlab/prot_bert", do_lower_case=False)

  >>> featurizer = BertFeaturizer(tokenizer)

  >>> feats = featurizer.featurize('D L I P [MASK] L V T', return_tensors="pt")

  >>> feats = featurizer.featurize('D L I P [MASK] L V T')

  Notes

  -----

  Examples are based on RostLab's ProtBert documentation.

  This class may contain code and/or documentation taken from the

  RobertaFeaturizer pull request (#2581), which have been moved here due to

  code restructuring.