packages: roms: download: help: add missing rootfs.img.

We already have a 'rootfs.img' argument but it is not documented in
the help.

This was broken from the start in the commit
9cc02ddde1 ("packages: roms: Start
adding automatic tests.").

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Acked-by: Adrien 'neox' Bourmault <neox@gnu.org>

resources/packages/roms/download

@@ -39,6 +39,10 @@ usage()
            "${progname}" \
            "grub.img" \
            "Get the path of the grub.img image"
+    printf "\t%s %s                      # %s.\n" \
+           "${progname}" \
+           "rootfs.img" \
+           "Get the path of the rootfs.img image"
     printf "\t%s <-h|--help>                     # %s.\n" \
            "${progname}" \
            "Display this help and exit"