Update malformed _toc.yml to fix jupyter-book build in CI

[lagru]

Running jupyter-book build . with v0.13.0 fails with the following message:

The Table of Contents file is malformed: toc is not a mapping: <class 'list'>
You may need to migrate from the old format, using:

	jupyter-book toc migrate _toc.yml -o _toc.yml

After updating _toc.yml accordingly, the build succeeds.

cc @emmanuelle

[lagru]

Intended as a quick fix. Long term we should probably aim for #66 or similar.

[lagru]

Aha, the CI fails because it now runs into API changes from scikit-image 0.18 to 0.19 (e.g. circle to disk, selem to footprint). I'll fix those errors in another PR. The original CI error seems fixed.

[lagru]

Just saw #65. :sweat_smile: Closing in favor of that one.