fix rag index names in eval_rag.py example
There was a mistake in the eval_rag.py parameters choices.

As specified in the rag configuration (see [documentation](https://huggingface.co/transformers/model_doc/rag.html?highlight=rag#transformers.RagConfig)), one can choose between 'legacy', 'exact' and 'compressed'. The legacy index is the original index used for RAG/DPR while the other two use the `datasets` library indexing implementation.

This issue was reported on the forum https://discuss.huggingface.co/t/rag-retriever-hf-vs-legacy-vs-exact-vs-compressed/2135/5