update documentation for pyterrier transformers

src/fast_forward/docs/pyterrier.md

@@ -3,7 +3,7 @@
 Fast-Forward indexes can seamlessly be integrated into [PyTerrier](https://pyterrier.readthedocs.io/en/latest/) pipelines using the transformers provided in `fast_forward.util.pyterrier`. Specifically, a re-ranking pipeline might look like this, given that `my_index` is a Fast-Forward index of the MS MARCO passage corpus:
 
 ```python
-bm25 = pt.BatchRetrieve.from_dataset(
+bm25 = pt.terrier.Retriever.from_dataset(
     "msmarco_passage", variant="terrier_stemmed", wmodel="BM25"
 )