mm, swap: correct comment in swap_usage_sub()

We will add si back to plist in swap_usage_sub(), just correct the wrong
comment which says we will remove si from plist in swap_usage_sub().

Link: https://lkml.kernel.org/r/20250222160850.505274-6-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>