ar71xx: fix typo in platform_do_upgrade_compex()

Fixes: a71742882855 ("treewide: use new procd sysupgrade $UPGRADE_BACKUP variable")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 63d611390c4d34a838e744e278529f30f9d2cc20)

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -164,7 +164,7 @@ platform_do_upgrade_compex() {
 
 	if [ -n "$fw_mtd" ] &&  [ ${fw_blocks:-0} -gt 0 ]; then
 		local append=""
-		[ -f "$UPGRADE_BACKUP" ] && append="-j $UPGRADE_BACKUPs"
+		[ -f "$UPGRADE_BACKUP" ] && append="-j $UPGRADE_BACKUP"
 
 		sync
 		dd if="$fw_file" bs=64k skip=1 count=$fw_blocks 2>/dev/null | \