doc: sandbox: replace sgdisk input with options

The input provided to sgdisk is in fact aimed for sfdisk. The use of
sgdisk and sfdisk, coming from different projects, is not the same.

So, this commit translates the sfdisk-formatted input into
sgdisk-compatible options. Partitions are not modified.

Signed-off-by: Corentin Guillevic <corentin.guillevic@smile.fr>
diff --git a/doc/arch/sandbox/sandbox.rst b/doc/arch/sandbox/sandbox.rst
index cd7f8a2..77ca6bc 100644
--- a/doc/arch/sandbox/sandbox.rst
+++ b/doc/arch/sandbox/sandbox.rst
@@ -388,7 +388,7 @@
 A disk image can be created using the following commands::
 
    $> truncate -s 1200M ./disk.raw
-   $> echo -e "label: gpt\n,64M,U\n,,L" | /usr/sbin/sgdisk  ./disk.raw
+   $> /usr/sbin/sgdisk --new=1:0:+64M --typecode=1:EF00 --new=2:0:0 --typecode=2:8300 disk.raw
    $> lodev=`sudo losetup -P -f --show ./disk.raw`
    $> sudo mkfs.vfat -n EFI -v ${lodev}p1
    $> sudo mkfs.ext4 -L ROOT -v ${lodev}p2