printk: fix comment for printk ratelimiting

The comment assumed the burst to be one and the ratelimit used to be named
printk_ratelimit_jiffies.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Cc: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>