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>
diff --git a/doc/board/AndesTech/ax25-ae350.rst b/doc/board/AndesTech/ax25-ae350.rst
index 89c485f..f795476 100644
--- a/doc/board/AndesTech/ax25-ae350.rst
+++ b/doc/board/AndesTech/ax25-ae350.rst
@@ -337,9 +337,11 @@
 Before building U-Boot SPL, OpenSBI must be build first. OpenSBI can be
 cloned and build for AE350 as below:
 
-git clone https://github.com/riscv/opensbi.git
-cd opensbi
-make PLATFORM=andes/ae350
+.. code-block:: none
+
+        git clone https://github.com/riscv/opensbi.git
+        cd opensbi
+        make PLATFORM=andes/ae350
 
 Copy OpenSBI FW_DYNAMIC image (build\platform\andes\ae350\firmware\fw_dynamic.bin)
 into U-Boot root directory