README: Fix CONFIG_SYS_NAND_MAX_DEVICE typo

This should be CONFIG_SYS_MAX_NAND_DEVICE. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Scott Wood <oss@buserror.net>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <york.sun@nxp.com>