replace markdown by common_mark when formatting is markdown
--- a/plugins/redmine_wiki_formatting/lib/rwf_application_helper_patch.rb
+++ b/plugins/redmine_wiki_formatting/lib/rwf_application_helper_patch.rb
@@ -49,7 +49,7 @@
       end
      elsif not pos2.nil? and pos2 < 20 then
       if pos2 < 20 then
-        formatting = "markdown"
+        formatting = "common_mark"
         text.slice!("formatting=markdown")
         text = Redmine::WikiFormatting.to_html(formatting, text, :object => obj, :attribute => attr)
       end
