doc: riscv: Update documentation for Sipeed MAIX boards

The MAIXDUINO runs fine with the sipeed_maix_bitm_defconfig but a different
board id parameter should be passed to kflash.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
1 file changed