fix doc example - AttributeError: type object 'RagModel' has no attribute...

fix doc example - AttributeError: type object 'RagModel' has no attribute 'from_question_encoder_generator_pretrained' (#15076)
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/models/rag/modeling_rag.py

@@ -305,7 +305,7 @@ class RagPreTrainedModel(PreTrainedModel):
         >>> from transformers import RagModel
 
         >>> # initialize a RAG from two pretrained models.
-        >>> model = RagModel.from_question_encoder_generator_pretrained(
+        >>> model = RagModel.from_pretrained_question_encoder_generator(
         ...     "facebook/dpr-question_encoder-single-nq-base", "t5-small"
         ... )
         >>> # saving model after fine-tuning