bug#45143: Incorrect markup in some doc/misc manuals

[Glenn Morris]

I felt like this report wasn't going anywhere, so I fixed it
myself in 3984044ad3..b3d310fa96 and will close this report.

I could not find a version of modus-themes.org that precisely
corresponded to the modus-themes.texi in Emacs.

If there are any bugs in the change, please followup in a new report.

Please also make new reports for any issues with the quality of the texi
files generated by Org. Ideally these would be against the org-mode package.