zram: correct documentation about sysfs node of huge page writeback

sysfs node for huge page writeback is writeback rather than write.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Link: https://lore.kernel.org/r/20200120102949.12132-1-zbestahu@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>