fix wiki-special link to "USB installation"

fix wiki-special link to "USB installation"

this was broken twice but in one rollout:
* one commit fixed the grammar from "Install from USB Stick" to
  "Install from a USB Flash Drive"
* then a followup comment renamed the whole thing "Prepare
  Installation Media" as also other medias where handled (shortly)

The wiki link breakage isn't really detctable until rolled out on
mediawiki, and it's a bit of an obscure thing anyway..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>