$ sudo apt-get update
$ sudo apt-get install redmine-plugin-markdown 

redmine-plugin-markdown is installed in your system.

Make ensure the redmine-plugin-markdown package were installed using the commands given below,

$ sudo dpkg-query -l | grep redmine-plugin-markdown *

You will get with redmine-plugin-markdown package name, version, architecture and description in a table

redmine-plugin-markdown package basic information: