systemd-nspawn(1): update example section

Remove the retired flag -d from Example 4. "Boot a minimal Arch Linux
distribution in a container". It has been retired here:
https://git.archlinux.org/arch-install-scripts.git/commit/pacstrap.in?id=0af6884aca68dcb7eed0b85fbc2960903df3d968

(cherry picked from commit 9a027075)