[Fix doc example] RagModel
# What does this PR do?

This part fails
https://github.com/huggingface/transformers/blob/623b4f7c63f60cce917677ee704d6c93ee960b4b/src/transformers/models/rag/modeling_rag.py#L308-L310

Change to `from_pretrained_question_encoder_generator` instead in this PR.

## Who can review?

@patrickvonplaten 