spatial/variograms errata: Wrapping, Gauss instead of Matérn, formatting

[falkmielke]

This PR corrects minor errors in the tutorial on spatial analysis / variograms.

• There was a putative error in the interval wrapping function; I have double checked the new one in a separate article (linked as footnote; but the link is optional -> feel free to remove).
• formatting: space missing before a quote
• reference to a wrong distribution function

This has very minor implications for the actual (randomly generated) data.
Therefore, I suggest to not re-calculate and re-commit the figure binaries.
(Instead, I have simply corrected the respective code lines.)

[falkmielke]

(The commit message says "wip", but no more progress is required: this is ready for review and merge. Thank you in advance!)