Typo in train test split of Link Prediction tutorial

## 📚 Documentation

Issue with link prediction tutorial

Hi, in the link prediction tutorial, it seems like there's a typo where you flipped the train and test sets.
for instance in "[Preparing train and test set](https://docs.dgl.ai/en/1.1.x/tutorials/blitz/4_link_predict.html#prepare-training-and-testing-sets)"
```
test_pos_u, test_pos_v = u[eids[:test_size]], v[eids[:test_size]]
train_pos_u, train_pos_v = u[eids[test_size:]], v[eids[test_size:]]
...
train_g = dgl.remove_edges(g, eids[:test_size])
```