scripts/checkstack.pl: add min_stack to the usage comment

After commit 572220aa ("scripts/checkstack.pl: Add argument to print
stacks greather than value."), it is appropriate to add min_stack to the
usage comment, then the users know explicitly that "min_stack" can be
specified like "arch".

Link: https://lkml.kernel.org/r/20231219125008.23007-3-yangtiezhu@loongson.cn


Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>