doc: fix issue with worker_threads docs

This example function returns a promise directly
rather than implicitly via async/await.

PR-URL: https://github.com/nodejs/node/pull/25712
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/worker_threads.md

@@ -25,7 +25,7 @@ const {
 } = require('worker_threads');
 
 if (isMainThread) {
-  module.exports = async function parseJSAsync(script) {
+  module.exports = function parseJSAsync(script) {
     return new Promise((resolve, reject) => {
       const worker = new Worker(__filename, {
         workerData: script