doc: Fix qemu-system-x86_64 example.

Reported by Fox <firefox@firemail.cc>.
Fixes <https://bugs.gnu.org/27052>.

* doc/guix.texi (Running GuixSD in a VM): Fix qemu-system-x86_64 example.

doc/guix.texi

@@ -7696,10 +7696,12 @@ Boot the USB installation image in an VM:
 @example
 qemu-system-x86_64 -m 1024 -smp 1 \
   -net user -net nic,model=virtio -boot menu=on \
-  -drive file=guixsd.img \
-  -drive file=guixsd-usb-install-@value{VERSION}.@var{system}
+  -drive file=guixsd-usb-install-@value{VERSION}.@var{system} \
+  -drive file=guixsd.img
 @end example
+The ordering of the drives matters.
+
 In the VM console, quickly press the @kbd{F12} key to enter the boot
 menu.  Then press the @kbd{2} key and the @kbd{RET} key to validate your
 selection.