Replaced mention of Heroku to Fly.io, as Heroku is not being used but Fly.io

Author: mayconblopes

src/content/4/en/part4d.md

@@ -317,7 +317,7 @@ When server-side sessions are used, the token is quite often just a random strin
 
 There have been many changes to the code which have caused a typical problem for a fast-paced software project: most of the tests have broken. Because this part of the course is already jammed with new information, we will leave fixing the tests to a non-compulsory exercise.
 
-Usernames, passwords and applications using token authentication must always be used over [HTTPS](https://en.wikipedia.org/wiki/HTTPS). We could use a Node [HTTPS](https://nodejs.org/api/https.html) server in our application instead of the [HTTP](https://nodejs.org/docs/latest-v8.x/api/http.html) server (it requires more configuration). On the other hand, the production version of our application is in Heroku, so our application stays secure: Heroku routes all traffic between a browser and the Heroku server over HTTPS.
+Usernames, passwords and applications using token authentication must always be used over [HTTPS](https://en.wikipedia.org/wiki/HTTPS). We could use a Node [HTTPS](https://nodejs.org/api/https.html) server in our application instead of the [HTTP](https://nodejs.org/docs/latest-v8.x/api/http.html) server (it requires more configuration). On the other hand, the production version of our application is in Fly.io, so our application stays secure: Fly.io routes all traffic between a browser and the Fly.io server over HTTPS.
 
 We will implement login to the frontend in the [next part](/en/part5).