README.md: fixed redirect and wget for Mac

Recent OS X versions come without wget included. You will need to use curl to fetch the archive.

I also noticed that the original url `https://github.com/telegeography/www.submarinecablemap.com/archive/master.zip` redirects to `https://codeload.github.com/telegeography/www.submarinecablemap.com/zip/master` so I changed that as well.