Here is a quick summary for downloading the dataset:
cd ~/fastai/courses/dl2
ln -s ~/data data && cd $_
mkdir pascal && cd $_
curl -OL http://pjreddie.com/media/files/VOCtrainval_06-Nov-2007.tar
curl -OL https://storage.googleapis.com/coco-dataset/external/PASCAL_VOC.zip
tar -xf VOCtrainval_06-Nov-2007.tar
unzip PASCAL_VOC.zip
mv PASCAL_VOC/*.json .
rmdir PASCAL_VOC