docs/system/riscv: Correct the indentation level of supported devices

The supported device bullet list has an additional space before each
entry, which makes a wrong indentation level. Correct it.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210430071302.1489082-5-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>