text: Title: Wrong link in `GDC` page
  Body: ### 📚 Describe the documentation issue
  
  Hi, there is a wrong link to the Diffusion Improves Graph Learning paper on `GDC` page.
  https://pytorch-geometric.readthedocs.io/en/latest/generated/torch_geometric.transforms.GDC.html
  
  
  
  ### Suggest a potential alternative/fix
  
  Can anyone update the link to [this one](https://arxiv.org/abs/1911.05485)?
  Thank you for reading this issue.