[Racket / rkt] Running the script fails

When running:

`racket learnracket.rkt`

with Racket v8.6 on Ubuntu (the current "latest" version), it stops with the following error:

```
learnracket.rkt:678:8: module: identifier already defined
at: out-port
in: (define-values (out-port) (open-output-file "/tmp/tmp.txt" #:exists (quote append)))
location...:
learnracket.rkt:678:8
```

Changing "out-port" to "out-port2" in lines 678 to 681 seems to fix the error but I am a total Racket beginner so I am not sure if that's indeed the correct fix.

Run `racket learnracket.rkt` with Racket v8.6 on Ubuntu and inspect `learnracket.rkt` lines 678-681, where the `out-port` definition reports a duplicate identifier. The work is done when the script runs without this error and the output-file behavior remains correct.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.