mm/slub: replace slub_$params with slab_$params in slub.rst

We've unified slab parameters with "slab_$params", then we can use
slab_$params in Documentation/mm/slub.rst.

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>