rockchip: MAINTAINERS: fix board name for Radxa ROCK 4C+

align with other ROCK series.

Fixes: 2b506407 ("rockchip: Add MAINTAINERS entry for Radxa Rock 4C+")
Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>