[Doc]fix ondisk_dataset_homograph.ipynb

[MikuSugar]

Description

When I was learning(https://docs.dgl.ai/stochastic_training/ondisk_dataset_homograph.html) to build the Graphbolt local data set, the data built according to the document tutorial code had an error when calculating the AUC in the link prediction task.
Fix data generation for lp_test_dode_pairs
"lp_test_node_pairs = edges[-num_tests, :] to lp_test_node_pairs = edges[-num_tests:, :] "
Refer to #7252

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[MikuSugar]

Hi ,@Rhett-Ying Thank you, I have resubmitted a PR as recommended.

[Rhett-Ying]

Thanks for your contribution