A typo in l07

[ppttzhu]

Hi professor, I found a typo in l07.ipynb, cell#: In [30]
prediction = filter_existing_edges(prediction, train_graph, author)[:7]
should be
prediction = filter_existing_edges(prediction, train_graph, author)[:6]
for top 6 predictions.

[aronwc]

fixed, thanks!