Update lint.go to add UTF8 as initialism

I think adding UTF8 would be good. ASCII is also in there and I think UTF8 should be either lowercase for a package name as in `unicode/utf8` or uppercase for a name as in `ErrInvalidUTF8`. (src: http://golang.org/src/pkg/regexp/syntax/parse.go#L40)

Some stats of the usage of 'UTF8' versus 'Utf8' in the stdlib:
http://golang.org/search?q=UTF8
http://golang.org/search?q=Utf8