binman: Use qemu-x86_defconfig in the example

The doc currently uses sandbox_defconfig as examples of enabling
debug/verbose output of binman. However during a sandbox build it
does not call binman at all. Change it to qemu-x86_defconfig.

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