Fix missing reference titles in retrieval evaluation of RAG
There's currently a bug in the retrieval evaluation script of RAG where for each sample, the first reference title is discarded.
Moreover only `k` reference titles were taken into account while we need all of them to compute the Precision @ k.

Because of this bug the index provided by the RAG team (hnsw M=128 + SQ8 with the "inner product to L2" trick) only had 35% of Precision @ k. With this fix it is back to 70%, which is consistent with what they had internally as far as I know.

Apparently the original parsing script used to add the question before the titles, that's why it was discarding the first entry.

cc @ola13 
Great!