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>

doc/README.x86

@@ -189,7 +189,7 @@ Offset   Description         Controlling config
 001000   me.bin              Set by the descriptor
 500000   <spare>
 700000   u-boot-dtb.bin      CONFIG_SYS_TEXT_BASE
-790000   vga.bin             CONFIG_X86_OPTION_ROM_ADDR
+790000   vga.bin             CONFIG_VGA_BIOS_ADDR
 7c0000   fsp.bin             CONFIG_FSP_ADDR
 7f8000   <spare>             (depends on size of fsp.bin)
 7fe000   Environment         CONFIG_ENV_OFFSET