doc: fix AX25-AE350 RISC-V documentation

Since commit 04883bf7 ("doc: update AX25-AE350 RISC-V documentation")
`make htmldocs` produces a log of warnings like

doc/board/AndesTech/ax25-ae350.rst:373:
WARNING: Block quote ends without a blank line; unexpected unindent.
doc/board/AndesTech/ax25-ae350.rst:0:
WARNING: Undefined substitution referenced: "_ __ ___ _ __ | (___ | |_) |".

Reformat the problematic passages.

Fixes: 04883bf7 ("doc: update AX25-AE350 RISC-V documentation")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>