docs: domain_support: Use capital letter for privilege modes

The RISC-V convention for the privilege mode is capital letter, like
'M-mode', instead of 'm-mode'.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>