Update README.md

dead link, they changed their dir structred and put the notebook in another branch.

README.md

@@ -28,6 +28,6 @@ tags:
 This is a dataset of prompts and solutions to the fill-in-the-middle (FIM) task
 presented in the [SantaCoder] paper.
 
-This dataset was generated using [this notebook](https://github.com/nuprl/MultiPL-E/blob/main/fill_in_the_middle/dataset_builder.ipynb).
+This dataset was generated using [this notebook](https://github.com/nuprl/MultiPL-E/blob/fim/fill_in_the_middle/dataset_builder.ipynb).
 
 [SantaCoder]: https://arxiv.org/abs/2301.03988