Please fix the go get url

[andreykyz]

change broken url -> go get -t -u sigint.ca/slice/cmd/preview
to good url -> go get -t -u sigint.ca/slice

[joeblew99]

@jnjackins
fails

 go get -t -u sigint.ca/slice
# sigint.ca/slice
../../../../../sigint.ca/slice/draw.go:63: undefined: primitive.Circle
../../../../../sigint.ca/slice/draw.go:64: undefined: primitive.Circle