Fix bug in the middlewares example (#2358)

Author: Qbject

docusaurus/docs/dev-docs/api/document-service/middlewares.md

@@ -347,7 +347,7 @@ Failing to do this will break the Strapi application.
 ```js
 const applyTo = ['api::article.article'];
 
-strapi.documents.use((context, next) => {
+strapi.documents.use(async (context, next) => {
  // Only run for certain content types
  if (!applyTo.includes(context.uid)) {
  return next();