Update mLinux reference.

Author: brocaar

docs/content/install/gateway/multitech.md

@@ -84,8 +84,8 @@ Example commands for upgrading / migrating:
 ```bash
 $ mkdir /var/volatile/flash-upgrade
 $ cd /var/volatile/flash-upgrade
-$ wget -O uImage.bin http://www.multitech.net/mlinux/images/mtcdt/3.3.24/uImage--3.12.27-r15.2-mtcdt-20180327203057.bin
-$ wget -O rootfs.jffs2 http://www.multitech.net/mlinux/images/mtcdt/3.3.24/mlinux-base-image-mtcdt-20180327203057.rootfs.jffs2
+$ wget -O uImage.bin http://www.multitech.net/mlinux/images/mtcdt/4.0.1/uImage--3.12.70-r20.2-mtcdt-20180620182056.bin
+$ wget -O rootfs.jffs2 http://www.multitech.net/mlinux/images/mtcdt/4.0.1/mlinux-base-image-mtcdt-20180620182056.rootfs.jffs2
 $ touch /var/volatile/do_flash_upgrade
 $ reboot
 ```