Add commentstring for pandoc

lua/autocommands.lua

@@ -7,6 +7,7 @@ vim.api.nvim_create_autocmd({ "BufEnter", "BufWinEnter" }, {
   group = markdown,
   callback = function()
     vim.o.filetype = 'pandoc'
+    vim.o.commentstring = '<!--%s-->'
     require('luasnip').filetype_extend("pandoc", { "markdown" })
     vim.o.spell = true
     require('spelling').spell_keymap()