/usr/lib/x86_64-linux-gnu/libmarkdown.so is in libmarkdown2-dev 2.1.8-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmarkdown.so.2.1.8
/usr/lib/x86_64-linux-gnu/libmarkdown.so.2.1.8