adding model card link direct in app.py

Same as was done in https://huggingface.co/spaces/CVPR/ml-talking-face/commit/e28a921859dfb71c94b6ee8392d1cce36d57fc4e, but different (I believe) than instructed in https://discuss.huggingface.co/t/how-to-link-the-spaces-with-arxiv/27094/4 and was initially done in [4e35c5794ff505f2b8d6d449111e4d1e5025f293](https://huggingface.co/spaces/emilylearning/spurious_correlation_evaluation/commit/4e35c5794ff505f2b8d6d449111e4d1e5025f293) for this demo. Perhaps my README link is formatted incorrectly.

Trying this way now, because it seemed to work for uncertainty demo when done here: https://huggingface.co/spaces/emilylearning/llm_uncertainty/commit/bcdc37328681f6605792739e1aa43a89b8b9011a, whereas I haven't yet successfully linked this spurious demo to arxiv.

app.py

@@ -1,3 +1,4 @@
+# Model card: https://huggingface.co/emilylearning/selection-induced-collider-bias
 # %%
 import gradio as gr
 import matplotlib.pyplot as plt