www: specify that the installation command should be run as normal user

I just linked the ghcup page to someone, but they ran it as root, thinking that it would perform a global install.

Maybe the website should specify that ghcup is an user-level installer and has to be installed and run as normal user.

A subtle way of doing this is adding a (non copy-able) $ before the command, like it's done in https://nixos.org/nix/ (click "get nix")