The default code here is broken in 1.0.0 but fixed in 1.0.1 at 11ty/e…

…leventy#2217

Swap to make the block work as-is with 1.0.0.

src/docs/languages/custom.md

@@ -151,10 +151,10 @@ const { marked } = require("marked");
 
 module.exports = function(eleventyConfig) {
   eleventyConfig.addExtension("md", {
-    compile: async function (inputContent, inputPath) {
+    compile: function (inputContent, inputPath) {
       let html = marked.parse(inputContent);
 
-      return async (data) => {
+      return function (data) {
         // Example: use `marked` only if useMarked is set in the Data Cascade
         if(data.useMarked) {
           return html;