Headings that contain `_` or `-` characters get broken links in the toc

[ahuffman]

You can test with this file if you wish: https://github.com/ahuffman/ansible-sudoers/blob/2.0.0/README.md

I find if I have headings with underscores or dashes the links in the toc does not get generated properly. Sometimes they're missing the underscores in the link. I manually update these typically as a work-around and turn off auto-updating the toc.