Update updating-convox.md
Update updating-convox.md

Update to use `convox rack` instead of `convox system`
sherbondy/convox.github.io,jjperezaguinaga/convox.github.io,jjperezaguinaga/convox.github.io,convox/site,bpo/convox.github.io,mojodna/convox.github.io,bpo/convox.github.io,convox/site,mojodna/convox.github.io,sherbondy/convox.github.io,convox/site,mojodna/convox.github.io,bpo/convox.github.io,convox/site,jjperezaguinaga/convox.github.io,sherbondy/convox.github.io