x86: doc: Change to use CONFIG_VGA_BIOS_ADDR

CONFIG_X86_OPTION_ROM_ADDR has been renamed to CONFIG_VGA_BIOS_ADDR.
Update the doc to refer to the new name.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>