doc: printf() codes: Fix format specifier for unsigned int

The format specifier for the "unsigned int" variable is documented as
"%d". However, it should be "%u". Thus, fix it.

Fixes: f5e90350 ("doc: printf() codes")
Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>