Use const instead of let for constant variables

Author: vejol

src/content/4/fi/osa4a.md

@@ -57,8 +57,8 @@ Ympäristömuuttujien käsittely on eriytetty moduulin <i>utils/config.js</i> va
 ```js
 require('dotenv').config()
 
-let PORT = process.env.PORT
-let MONGODB_URI = process.env.MONGODB_URI
+const PORT = process.env.PORT
+const MONGODB_URI = process.env.MONGODB_URI
 
 module.exports = { MONGODB_URI, PORT }
 ```