doc: fix StarFive VisionFive v2 documentation

The number of the partition that U-Boot SPL loads the main U-Boot from is
defined as 2 by CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION=0x2. The
partition type GUID is not used currently.

Reword the description of the boot process to make it clearer.

Fixes: 5ecf9b0b ("board: starfive: add StarFive VisionFive v2 board support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>