slub: fix slub_max_order Documentation

slub_max_order default is 3 (aka PAGE_ALLOC_COSTLY_ORDER), not 1
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

Documentation/vm/slub.txt

@@ -117,7 +117,7 @@ can be influenced by kernel parameters:
 
 slub_min_objects=x		(default 4)
 slub_min_order=x		(default 0)
-slub_max_order=x		(default 1)
+slub_max_order=x		(default 3 (PAGE_ALLOC_COSTLY_ORDER))
 
 slub_min_objects allows to specify how many objects must at least fit
 into one slab in order for the allocation order to be acceptable.