[Documentation] specify <platform> types

In this file
https://github.com/CCExtractor/ccextractorfluttergui/blob/master/INSTALL.MD

Replace line 18
```
- Next enable the flutter platform specific flag with `flutter config --enable--desktop`, more info on this [here](https://flutter.dev/desktop)
```

With this:
```
- Next enable the flutter platform specific flag with `flutter config --enable--desktop`, being ``: `windows`,`macos`,`linux`. More info on this [here](https://flutter.dev/desktop)
```

the link here does not explain directly any of that, maybe the webpage has changed.

I see it's mentioned more than once, so add some explanation.