examples on nbviewer returns 404

[clausmichele]

Describe the bug
The link to the example notebooks is broken: https://nbviewer.org/github/python-visualization/folium/tree/master/examples/

[ocefpaf]

Describe the bug The link to the example notebooks is broken: https://nbviewer.org/github/python-visualization/folium/tree/master/examples/

We should update it to reflect the master to main move. Try https://nbviewer.org/github/python-visualization/folium/tree/main/examples/, it should work.