Fix token_type_id in BERT question-answering example
`token_type_id` wasn't being set correctly in the code examples for BERT Question answering. It's being turned into the sequence embedding, hence needs to highlight whether each token belongs to sequence 0 or 1.

For this small example the model returns the correct answer even though the parameter was incorrectly set, but for bigger paragraphs that is not the case.

I changed the code to use encode_plus which returns the correct `token_type_id`.