vsprintf: Update %pGp documentation about that it prints hex value

The commit 23efd080 ("vsprintf: Make %pGp print
the hex value") changed the behavior of %pGp printk format.
Update the documentation accordingly.

Fixes: 23efd080 ("vsprintf: Make %pGp print the hex value")
Reviewed-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/YXlKqCPY9suM4mfT@alley