Documentation: minor update to the tutorial about Ubuntu as SOS

Minor update to the tutorial on how to use Ubuntu as the Service OS
with ACRN. Specifically, the instructions for making the Grub menu
visible at boot time now involved modify the /etc/default/grub file
and not the /boot/grub/grub.cfg as before.

Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

doc/tutorials/using_ubuntu_as_sos.rst

@@ -188,20 +188,19 @@ You can download latest Service OS kernel from
         The command line for the kernel in /etc/grub.d/40_custom should be all
         as a single line, not as multiple lines. Otherwise the kernel will fail to boot
 
-   * Update Grub on your system
+   * Modify the ``/etc/default/grub`` file to make the grub menu visible when booting.
+     There are a couple of lines to be modified, as shown below.
 
      .. code-block:: none
 
-        sudo update-grub
+        #GRUB_HIDDEN_TIMEOUT=0
+        GRUB_HIDDEN_TIMEOUT_QUIET=false
 
-     At this point, you need to modify ``/boot/grub/grub.cfg`` file manually to
-     enable the timeout so that the system has an opportunity to show you the
-     grub menu. (Without this the grub choice menu won't display.)
+   * Update Grub on your system
 
      .. code-block:: none
 
-        #set timeout_style=hidden
-        set timeout = 10
+        sudo update-grub
 
 #. Reboot the system