doc: fix opensbi build steps for AX25-AE350

OpenSBI build steps are marked as normal text in
AX25-AE350 documentation.

Move them into code-block so-that it can show it
as build steps.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>