docs: printk-formats: Treat char as always unsigned

The Linux kernel switched to have char be equivalent to usigned char.
Reflect this in the printk specifiers.

Fixes: 3bc753c0 ("kbuild: treat char as always unsigned")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230703145839.14248-2-andriy.shevchenko@linux.intel.com