update linux 9p image mount hook doc

should be host9p, otherwise the initcpio hook will try to mount
/dev/sda1 (or whatever you have configured) over /new_root as a 9p
fs (which it isn't) and the system will complain with "no channels
available" and drop you in the emergency shell. Instead, host9p
should be mounted over /new_root as type 9p.

docs/linux-9p-image.md

@@ -31,7 +31,7 @@ run_hook() {
 
 mount_9p_root() {
     msg ":: mounting '$root' on real root (9p)"
-    if ! mount -t 9p "$root" "$1"; then
+    if ! mount -t 9p host9p "$1"; then
         echo "You are now being dropped into an emergency shell."
         launch_interactive_shell
         msg "Trying to continue (this will most likely fail) ..."