doc: printk-formats: fix format specifier to %p[Ii]4[hnbl]

This might have been an oversight, as '4' is needed as %pI4/%pi4
are format specifiers and [hnbl] are extensions of it.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
CC: Joe Perches <joe@perches.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>