Works for me. Unless its a real problem with github, which it may be but I’m going to guess that its not, its probably a problem with your connection.
Can you get to the same URL via a browser?
https://raw.githubusercontent.com/Homebrew/install/master/install
If you get an error there too, you know what your problem is. (Talk to your network admin)
If that works…maybe try doing just the “curl” command in your terminal to see if curl gives a more specific error?
Also, if that does work, save that file to a file on your computer (brew_install.rb) and then run it via
ruby brew_install.rb