Add mandatory system update to install instructions

[RivenSkaye]

Earlier this week I set up Arch WSL on my system at the office, all was well. Today I decided I might as well run it on my laptop too, except running sudo pacman -S base-devel was enough to leave the system in a broken and unusable state. I reinstalled in another directory and redid the initial setup, ensuring a sudo pacman -Syu before installing base-devel, received an update for glibc, and all was well. I'm not entirely sure what goes wrong, maybe something in the base-devel dependency chain is causing Arch WSL to lose track of its glibc version? I do notice an updated glibc (2.36-3) being pulled in with the sys update, and sudo was complaining about needing anything >= 2.34 after the failed attempt.

Anyways, small mistake that requires redoing more manual effort than I'd care for repeating