Fix snek5000/docs/examples/scripts/tuto_cbox.py

[paugier]

See snek5000/snek5000#167 (comment)

I guess the issue comes from snek5000-cbox. We need to understand what happens.

[akhoubani]

I noticed this problem:

snek5000-cbox/src/snek5000_cbox/cbox.usr.f

Line 282 in 921a975

if (stretch_x.ne.0.0) then

should be if (stretch_y.ne.0.0) then. It means this version does not stretch the mesh at all.

[paugier]

Does it fix the issue with the tuto script?

Anyway, it is much better to do a tiny commit with only this modif and to propose a PR with written in its description "Fixes #32" so that this issue would be closed automatically when the PR is merged.