doc: Chromebook Coral: Fix typo for "Top of CAR region"

The value for "Top of CAR region" should be fefc0000, not
fefc000. This matches the Kconfig default values, as
SYS_CAR_ADDR and SYS_CAR_SIZE are 0xfef00000 and 0xc0000
respectively.

Signed-off-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>