[Doc] Fix a typo in the tutorial for link prediction (#2928)

Fix a typo in the tutorial for "Link Prediction using Graph Neural Networks".
Co-authored-by: Jinjing Zhou <VoVAllen@users.noreply.github.com>

tutorials/blitz/4_link_predict.py

@@ -99,7 +99,7 @@ neg_u, neg_v = np.where(adj_neg != 0)
 
 neg_eids = np.random.choice(len(neg_u), g.number_of_edges())
 test_neg_u, test_neg_v = neg_u[neg_eids[:test_size]], neg_v[neg_eids[:test_size]]
-train_neg_u, train_neg_v = neg_u[neg_eids[test_size:]], neg_v[neg_eids[test_size:]]
+train_neg_u, train_neg_v = neg_u[neg_eids[train_size:]], neg_v[neg_eids[train_size:]]
 
 
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