Replacing http://goo.gl links with https://goo.gl links

If you wget or curl the http://goo.gl links, like the instructions say, a man-in-the-middle attacker could hijack that  request and redirect to it a malicious version of crouton, e.g. one that installs a rootkit after installing Linux. But the goo.gl service supports and works fine with https, so there's no reason not to tell people to use that instead.