Update docs with curl cmdline which follows redirects.

To improve security github has moved all user content from the github.com
domain to githubuserconent.com domain, see
https://developer.github.com/changes/2014-04-25-user-content-security/ for more
information.

They offer redirects from the original locations, but curl won't follow
redirects without the -L option.

An alternative would be to update the `https://raw.github.com/` URLs to be
`https://raw.githubusercontent.com/` instead but if they ever change the domain
again you'll need to update the documentation again.