Small typo on optimizer

In your notebook, you follow the [official pytorc_geometric tutorial](https://pytorch-geometric.readthedocs.io/en/latest/notes/introduction.html) regarding the GNN but you wrongly copy-pasted the `weight_decay` for the optimizer. Indeed `weight_decay=5e-3` is very big and should be reduced to `weight_decay=5e-4` which doubles the accuracy of the model.