mxgrub: mxgrub suggest to run "--update" when "--update" is actually used.

[donald]

buczek@chimichangas:~$ sudo mxgrub --update

  Note: GRUB menu and installed kernels differ.

    0 Kernel(s) from the menu are missing in '/boot'.
    2 Kernel(s) installed aren't listed in the menu.

  Consider a rewrite of grub.cfg (and mind deinemuddah.molgen.mpg.de):

           /usr/sbin/mxgrub --update

/boot/grub/grub.cfg: updated