Fix results_short

[zirlman]

Hi Patrick,

first of all, thank you for the nice notebook. It really helped me to figure out how to load large documents into the Longformer model. However, when running the notebook in Colab I've found a small bug, namely in cell 38 you've written

results_short = results_short.map(evaluate)

But I think you've intended to write

results_short = short_validation_dataset.map(evaluate)