2 typos in modeling_rag.py
# What does this PR do?
Fix 2 typos in `modeling_rag.py`
`from_encoder_generator_configs` --> `from_question_encoder_generator_configs`

## Who can review?
@lhoestq

Hi! Could you run `make style` on your branch so that the code quality check passes? Thanks!<|||||>Hi guys, I only have mobile phone until Dec. 1. I will do it as soon as I can access PC.<|||||>@lhoestq @LysandreJik done applying style. Sorry for late!