Fix formatting in man page

Fixes #815

groff 1.23.0 changed the way '-', '~', and '^' characters are rendered.

See:

- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1052675
- https://man7.org/linux/man-pages/man7/man-pages.7.html
  (search for "Generating optimal glyphs")