correct the Arch upgrade instructions for prerelease
correct the Arch upgrade instructions for prerelease

The `sudo pacman -Syu` command assumes that you're using the (official!)
Arch package, but if you're running prerelease that won't update you.
Suggest a more appropriate command if the current client is prerelease.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client