[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.

Abre INSTALL.MD e inspecciona la línea 18 y la otra aparición del Flutter desktop platform flag mencionada en el issue. Actualiza la documentación para identificar windows, macos y linux como valores de plataforma válidos, y verifica que cada explicación relevante sea coherente con la redacción solicitada.

Escrito por el modelo de indexación a partir del texto del issue.