fix a trivial typo in scripts/checkstack.pl

Trivial change in a comment.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Andre Haupt <andre@finow14.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>