Update upload-images.md [replace `in` by `of` for multiple files upload]

[Sequoya42]

Replace in with of

[ [ 1, 'toto' ], [ 2, 'tata' ] ]
> for (z of x) { console.log(z)}
[ 1, 'toto' ]
[ 2, 'tata' ]
undefined
> for (z in x) { console.log(z)}
0
1```

Otherwise, trying to access img[0] does not make sense.

[ygj6]

Thank you.