Add UTF-8 BOM to votl_cheatsheet.txt

Well, my change that you just merged worked fine until I ran :helptags. It turns out that :helptags complains about mixed encodings between help files. I'm not sure why that would be, but it's easy enough to fix by adding the UTF-8 BOM to votl_cheatsheet.txt as well. There's the slight added bonus of now being able to add Unicode text to that file without issue.