fixed docs for transformers question answering annotaters

[ahmedlone127]

Fixed docs for transformer question answering annotators

Description

All questions Answer Transformers use

documentAssembler = MultiDocumentAssembler() \
    .setInputCols(["question", "context"]) \
    .setOutputCols(["document_question", "document_context"])

But the docs are wrong and say

documentAssembler = MultiDocumentAssembler() \
    .setInputCols(["question", "context"]) \
    .setOutputCol(["document_question", "document_context"])