Fix typo in github link, and make link point to specific branch

README.md

@@ -11,7 +11,7 @@ tags:
 
 This model contains the GraphCast checkpoints created as part of [(Subich 2024)](https://arxiv.org/abs/2408.14587), which fine-tunes the "standard" GraphCast ¼°/37-level model on the 2019-2021 period.  The primary goal of the study was to adapt the model to the Canadian GDPS analysis, but another product produced along the way was a "control" version trained on ERA5 data, which is more widely available.
 
-The model's training code is available at [https://github.com/csubich/graphcast][https://github.com/csubich/graphcast].
+The model's training code is available at [https://github.com/csubich/graphcast](https://github.com/csubich/graphcast/tree/graphcast_train).
 
 The model checkpoints are in the `params/ar{1,2,4,8,12}` directories, each directory noting the number of autoregressive forecast steps completed.  See the arxiv paper for details about the training schedule.  The respective `era5.ckpt` files are the model versions trained on ERA5 data, and the `gdps.ckpt` files are those trained on the GDPS analysis data.  The `ar12` checkpoints are the final result of training, and the earlier ones are provided for research \& reference.