MINOR: Tweak headings in 3D tutorail for compat with docs.

[danielballan]

@caspervdw I manually edited the notebook file to change your "heading" cells to "markdown" cells with h2-style headings. The pages doc/index.rst and doc/tutorials.rst automatically generate links from h1 headings.

[caspervdw]

Thanks for making it compatible with the rest of the docs.