tools/vm/slabinfo: align usage output columns

Usage message uses spaces not tabspaces, a few tabspaces have snuck in
making the columns not align correctly when output.

Align usage output columns using spaces instead of tabspaces.

Link: http://lkml.kernel.org/r/20190212001219.27769-4-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>