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>

tools/binman/README

@@ -931,13 +931,13 @@ old.
 To enable a full backtrace and other debugging features in binman, pass
 BINMAN_DEBUG=1 to your build:
 
-   make sandbox_defconfig
+   make qemu-x86_defconfig
    make BINMAN_DEBUG=1
 
 To enable verbose logging from binman, base BINMAN_VERBOSE to your build, which
 adds a -v<level> option to the call to binman:
 
-   make sandbox_defconfig
+   make qemu-x86_defconfig
    make BINMAN_VERBOSE=5