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: 3bc753c ("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

Author: andy-shev

Documentation/core-api/printk-formats.rst

@@ -15,8 +15,9 @@ Integer types
 
 	If variable is of Type,		use printk format specifier:
 	------------------------------------------------------------
-		char			%d or %hhx
+		signed char		%d or %hhx
 		unsigned char		%u or %x
+		char			%u or %x
 		short int		%d or %hx
 		unsigned short int	%u or %x
 		int			%d or %x