Fix typo in kNN example.

README.md

@@ -122,7 +122,7 @@ edge_index = knn_graph(x, k=2, batch=batch, loop=False)
 ```
 print(edge_index)
 tensor([[0, 0, 1, 1, 2, 2, 3, 3],
-        [1, 2, 0, 2, 0, 3, 1, 2]])
+        [1, 2, 0, 3, 0, 3, 1, 2]])
 ```
 
 ## Radius-Graph