Update pkg_building.Rmd

[osorensen]

The link to the code chapter in The R Packages Book did not work, presumably because of the uppercase in Code.html. With this modification it works.

[mpadge]

Thanks @osorensen, but that error has already been fixed. Link from current version is to /r.html, which is why it doesn't work. Updated version is now /code.html, but case-insensitive, so Code.html also works. Current development version can be seen at https://devdevguide.netlify.app/pkg_building#code-style, where you can see that the link works. I'll close this now, but thanks for keeping a keen eye on thing anyway, and feel free to PR anything else you may notice 👍