Update part3b.md (add note on fly.io deployment error)

Author: cats256

src/content/3/en/part3b.md

@@ -163,6 +163,21 @@ Fly.io creates a file <i>fly.toml</i> in the root of your app where the app is c
 
 We have now defined in the part [env] that environment variable PORT will get the correct port (defined in part [services]) where the app should create the server. Note that the definition might be already there, but some times it has been missing.
 
+Note: For some users, the fly.toml file generated by fly.io will have this configuration:
+
+```bash
+[http_service]
+  internal_port = 3000
+```
+
+This will cause the following warning once deployed and result in your app being inacessible:
+
+```bash
+WARNING The app is not listening on the expected address and will not be reachable by fly-proxy
+```
+
+Change internal_port to 8080 if your configuration is as mentioned above.
+
 We are now ready to deploy the app to the Fly.io servers. That is done with the following command:
 
 ```bash