Revert "Revert "update sub-headers in 23_tutorial.vision.ipynb (#2800)" (#2801)" (#2802)
Revert "Revert "update sub-headers in 23_tutorial.vision.ipynb (#2800)" (#2801)" (#2802)

This reverts commit 5fbb9acea497562e81bf940a6920fb855aa814f8.
fastai/fastai