README: fix Archlinux installation instruction

[qwertviop]

Using pacman -Sy <pkg> to install a package may lead to a partial upgrade and should not be used. Use pacman -S <pkg> instead. See https://wiki.archlinux.org/title/Pacman#Installing_packages.

[JohnAZoidberg]

Oh, I've done it wrong my whole life

[qwertviop]

I mean, if you keep your system up to date or run pacman -Syu before doing it, it is very unlikely to cause any issues, but if you for example don't update your system for a couple of months, and then install something with -Sy chances are you will run into dependency issues or other breakage. So you should just use -S <pkg> or -Syu <pkg> and be on the safe side.