Hello and thanks for reporting this issue.
The error occurs because another plugin, or possibly even a theme, is loading an older version of Parsedown on your site.
If you want to find out which plugins use Parsedown, assuming you have shell access to your WordPress, you can do:
find /path/to/wordpress -name Parsedown.php
If it’s a plugin you don’t need you can uninstall it. Or you can tell the developer to update their plugin to the latest Parsedown.
Fortunately the documentation is markdown, so you can read it with any text editor or markdown editor: The documentation is in the docs folder of each plugin.
For the main plugin, you can also read the documentation online via the github pages:
I will see if I can make the plugin not require ParsedownExtra in the next release.
Hope this helps. Please let me know if you have more questions.