printk.c: comments should refer to /proc/vmcore instead of /proc/vmcoreinfo

In one of those comments a typo was fixed, too.

Signed-off-by: Dirk Gouders <dirk@gouders.net>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>