fix download uefi-ntfs.img

[FuPeiJiang]

download the FILE, not the folder(I think it downloads the html page https://github.com/pbatard/rufus/tree/master/res/uefi)

change:
https://github.com/pbatard/rufus/raw/master/res/uefi/
to
https://github.com/pbatard/rufus/raw/master/res/uefi/uefi-ntfs.img

[WaxyMocha]

Ok so I checked this part of code and turns out that in general, I don't think it does anything. You are right that current code downloads html instead of image, but then it is copied to /dev/sdx2 and that doesn't make sense. One more reason to refactor the whole thing.
Thanks for PR :)

[WaxyMocha]

No, wait. If it's just raw image, it does make sense to copy it directly to block device.

[FuPeiJiang]

this works
sudo dd if="/path/to/uefi-ntfs.img" of=/dev/sdX2
but it's dd, so it's dangerous,

python does a copy, which is less dangerous than overwrite, but it would require mounting "/path/to/uefi-ntfs.img" AND mounting /dev/sdX2

[FuPeiJiang]

I have more commits,

but since: CONTRIBUTING.md

Create commits based on minimal independent changes
Avoid creating commits that do multiple things at once as this will help other developers understand the change history.

I guess pull requests shouldn't contain unrelated stuff, so I started with this one