Data formatting step outputs incorrectly keys for JSONL

[ridoy]

https://github.com/openai/evals/blob/main/docs/build-eval.md

Reproduction steps:
Have a csv with prompt and completion columns
Run openai tools fine_tunes.prepare_data -f data.csv
Inspect output JSONL file and verify entries have prompt and completion keys but not the required input key

It may very well be that the csv column names should be different, but prepare_data complains about the input not having prompt and completion columns. It would be helpful if the docs explained what columns are required.