Revert change to fNIRS tutorial

[rob-luke]

In #9843 I initially changed the default value of the beer lambert law, but then we had to leave it the same for a depreciation cycle. Somehow this change snuck through, this causes all the plots to have no data in the tutorial as reported at https://mne.discourse.group/t/fnirs-tutorial-does-not-display-visualization

in the coming weeks I will complete the transition to the new default. Can we back port this PR?

[drammock]

tutorial looks reasonable except for the first figure, which is just a big black square
https://38736-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/preprocessing/70_fnirs_processing.html?highlight=functional%20infrared

[rob-luke]

Thanks for posting the link and checking the output @drammock The figures now look as they should. I think we can merge if youre happy @drammock @larsoner @agramfort

The black figure occurs before the code change in this PR, so i think is unrelated and we can merge.
The black figure should be a 3d image, so I guess something has changed with the 3d plotting dependencies or MNE code? I have seen a similar thing when trying to use the github actions CI, but not with circle.

[agramfort]

backported to 0.24 maint branch

[rob-luke]

Thank you