Update part4a.md

new index.js use app.listen instead of server.list after the imports

Author: danielyepes32

src/content/4/es/part4a.md

@@ -83,7 +83,7 @@ const app = require('./app') // la aplicación Express real
 const config = require('./utils/config')
 const logger = require('./utils/logger')
 
-server.listen(config.PORT, () => {
+app.listen(config.PORT, () => {
   logger.info(`Server running on port ${config.PORT}`)
 })
 ```