textproc/ruby-kramdown: update to 2.4.0

2.4.0 (2022-04-25)

A minor release bringing a small enhancement to the kramdown converter and
some bug fixes.

Changes

* 1 minor change:

- Allow using a different list indentation (pull request #753 by
          Carsten Borman)

* 2 bug fixes:

- Fixed escaping of colon when converting to kramdown (fixes #751 by
          Carsten Borman)

- Fixed setting highlighter class for code spans in HTML converter
          when using a default language (fixes #752 by Aswhin Maroli)

(taca)