ar71xx: fix typo in platform_do_upgrade_compex()

Fixes: 641f6b6c26cb ("treewide: use new procd sysupgrade $UPGRADE_BACKUP variable")
Signed-off-by: Rafał Miłecki <[email protected]>

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 | \