[file_selector_web] update documentation

I was following the installation guide for [file_selector_web](https://pub.dev/packages/file_selector_web) plugin and I think it needs tome update.

The docs says to add those two dependencies but they don't work:
```
file_picker: ^0.7.0
file_picker_web: ^0.7.0
```

Instead they work with:
```
file_selector: ^0.7.0+2
file_selector_web: ^0.7.0+1
```


Cc @ditman 