Clarify express-async-errors import order

Clarify that express-async-errors must be imported before routes are defined, in order for the library to work correctly, as explained in the docs (https://github.com/davidbanham/express-async-errors#usage).

Author: Jon Rutter

src/content/4/en/part4b.md

@@ -832,7 +832,7 @@ npm install express-async-errors
 ```
 
 Using the library is <i>very</i> easy.
-You introduce the library in <i>app.js</i>:
+You introduce the library in <i>app.js</i>, _before_ you import your routes:
 
 ```js
 const config = require('./utils/config')