[arch-releng] mention `pacman -Syu` in official install guide

[Andre "Osku" Schmidt]

hi,

just noticed that theres no mention at all about `pacman -Syu` in
https://wiki.archlinux.org/index.php/Official_Arch_Linux_Install_Guide

and now that we can do `pacman -Syu foobar` i would think it should be
the first(ish) command to tell the users?

anyone good with words? ;)

cheers
.andre

ps. or is there a reason not to have it mentioned in this guide?