Fix typos.

[geoffrey-eisenbarth]

I also discovered that there's a clash between --main-font and --primary-font, where

1. src/main.css
2. src/variables.css
3. www/docs/60-variables.md
4. www/_includes/layout.vto

use --main-font (and --secondary-font), and

1. src/utils.css
2. www/docs/80-utils.md

use --primary-font.

This change doesn't seem to be documented in any of the www/release/*.md files, like the switch from --display-font to --secondary-font is (and actually, just checking this I see there's still a holdover where www/docs/80-utils.md contains an entry for --display-font).

If someone could advise on whether to use --main-font or --primary-font, I can add those fixes (and the --display-font one) to this PR.

[dz4k]

--main-font is the preferred one.

I would like --secondary-font and --display-font to coexist, with --display-font defaulting to var(--secondary-font). --secondary-font would be for UI elements and figure captions, and --display-font for headings.