[BUG] Contours in plot_3d_map_to_surface_projection examples are not being rendered correctly

[ymzayek]

BTW while checking this I noticed that in the same example where plot_surf_contours is used the contours are not rendered anymore. This is not related to this PR as I can also see it in the build artifacts dating back at least a month. I'll open a separate issue

Originally posted by @ymzayek in #3949 (comment)

Edit: If you look closely you can see small points, so it seems the rendering changed rather than being non existent.

See stable docs: https://nilearn.github.io/stable/auto_examples/01_plotting/plot_3d_map_to_surface_projection.html

See dev docs: https://nilearn.github.io/dev/auto_examples/01_plotting/plot_3d_map_to_surface_projection.html

[ymzayek]

It seems to be coming from this merge 069980c