Hi,
It's a false positive. We've double checked that the demo files haven't been tampered with, so unless someone hacked itch.io itself during the time you downloaded the demo, there's no way there was a virus in the files.
That is, assuming you downloaded the demo from itch.io; we haven't uploaded the demo anywhere else so any mirrors are unofficial and subject to whatever the uploader has done to them.
Here's an online virus test that shows two false positives and an otherwise clean list, if you're interested: https://www.virustotal.com/gui/file/8b06db8be4781282ef7ec7663dfd8ad2606d864d4c4c02f3fbd5f0cd0e99491f/detection