text: Title: chooses 15% of token
  Body: From paper, it mentioned 
  > Instead, the training data generator chooses 15% of tokens at random, e.g., in the sentence my
  > dog is hairy it chooses hairy. 
  
  It means that 15% of token will be choose for sure. 
  
  From https://github.com/codertimo/BERT-pytorch/blob/master/bert_pytorch/dataset/dataset.py#L68, 
  for every single token, it has 15% of chance that go though the followup procedure. Does it aligned with 15% of token will be chosen? 