Fix broken 'uses-values' links in the Porter's Handbook

The USES chapter sets an xml:id on its title tag but this doesn't
seem to generate an anchor. As a consequence there currently are
several 'uses-values' xrefs that point to nowhere.

This is only really noticeable in the single page HTML version
(book.html) of the handbook.

PR: 227123
Reported by: 0mp
Approved by: mat
Differential Revision: https://reviews.freebsd.org/D15642