More efficient wordcount.lua example.

doc/lua-filters.md

@@ -408,8 +408,7 @@ words = 0
 wordcount = {
   Str = function(el)
     -- we don't count a word if it's entirely punctuation:
-    local s = el.text:gsub("%p","")
-    if #s > 0 then
+    if el.text:match("%P") then
         words = words + 1
     end
   end,