contributing: Fix lua stript

The lua script for filtering out toctrees started to fail with the latest pandoc with:

./CONTRIBUTING.lua:12: attempt to index a nil value (field 'integer index')

We fix it by using the classes:includes which is simpler and it works.

contributing/CONTRIBUTING.lua

@@ -8,9 +8,8 @@
 -- tool. This is needed when converting a set of reST documents to a markdown
 -- one where directives like `toctree` and `contents` are not supported and end
 -- up translated literally.
-function Div(el)
-  local class = el["c"][1][2][1]
-  if class == "toctree" or class == "contents" then
+function Div(div)
+  if div.classes:includes('toctree') or div.classes:includes('contents') then
     return {}
   else
     return nil