Update app.py

app.py

@@ -139,7 +139,7 @@ with gr.Blocks(theme=theme, title="XRD Pattern Generator") as demo:
         """
         #  XRD Pattern Simulator from CIF
         Upload a Crystallographic Information File (.cif) to generate its simulated
-        X-ray Diffraction (XRD) pattern using pymatgen.
+        X-ray Diffraction (XRD) pattern using [pymatgen](https://github.com/materialsproject/pymatgen).
         """
     )