Fix broken mybinder links for dask-examples notebooks

[GenevieveBuckley]

All the binder badge links are broken, this PR restores that functionality.

They broke for two reasons:

1. The dask-examples "main" branch was recently renamed (previously named "master"), and
2. A backwards incompatible change happened in the way binder generates/parses links (see here for more details)

Neither of these should be a problem going forward. However, we will need to be careful with the way people generate binder badges for dask-examples - if you use https://mybinder.org/ it will give you a link that only works for newer versions. We can either:

• Use the link formatting with urlpath like in this PR, or
• Update the pinned dependencies over at dask-examples to be more modern

Cross reference: dask/dask-examples#196

[GenevieveBuckley]

(I'm not going to wait for discussion before merging, it's fixing a problem so we probably don't want to leave the links broken for longer than they need to be. I've checked the updated links individually & they all seem to be working now.)