Fix image URL for MMiDS textbook in gallery (#1289)

Author: sebroc

docs/gallery.yml

@@ -13,7 +13,7 @@
 - name: Mathematical Methods in Data Science (with Python)
   website: https://mmids-textbook.github.io/
   repository: https://github.com/MMiDS-textbook/MMiDS-textbook.github.io
-  image: https://github.com/MMiDS-textbook/MMiDS-textbook.github.io/blob/main/mmids-cover.png
+  image: https://raw.githubusercontent.com/MMiDS-textbook/MMiDS-textbook.github.io/main/mmids-cover.png
 - name: "Unraveling Hidden Patterns of Brain Activity: A Journey Through Hemodynamic Deconvolution in Functional MRI"
   website: https://phd.enekourunuela.com
   repository: https://github.com/eurunuela/phd