`Additional Resource` and `Homework` hyperlinks broken

[athchen]

The Additional Resources and Homework hyperlinks in the TOC are broken for each section after chapter 2.

For example, Homework in chapter 3 (Section 3.5) and chapter 4 (Section 4.12) link to the Homework in chapter 2 (section 2.7).

Looks like this is a problem with the pandoc headers and is fixed in the developmental version of bookdown: https://github.com/rstudio/bookdown/issues/909