i made it confusing; it is  with your own text data;
are you trying to fine-tune for a certain task as @lewtun mentioned and then there are examples in the link he provided;
or are you trying to fine-tune BERT (as language modeling) with domain-specific vocab/text to adjust for a certain domain - for this you can use the first example in  his link (language model): transformers/examples/language-modeling at master · huggingface/transformers · GitHub 3