tools/vm/slabinfo: clean up usage menu debug items

Attempt to make the usage comment for debug options a little cleaner.

Link: http://lkml.kernel.org/r/20190212001219.27769-5-tobin@kernel.org


Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>