[css-values] Font-relative lengths section heading is missing 'cap'

[cvrebert]

The heading for https://drafts.csswg.org/css-values/#font-relative-lengths neglects to mention the newly-added 'cap' unit.

5.1.1. Font-relative lengths: the em, ex, ch, ic, rem, lh, rlh units

[tabatkins]

Merged #1134 to fix this.