sysutils/unetbootin: amend MASTER_SITES, fix minor indentation bug

As of r471558 (now commit a7f0cdea95a2) the project had moved from
SourceForge to GitHub, yet distfiles are provided at both locations.
Prefer the former by default to enjoy its vast selection of mirrors
while downloads from the latter are less reliable in certain parts
of the world (e.g. I'm getting truncated files off it all the time).