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

Documentation/admin-guide/blockdev/zram.rst

@@ -319,7 +319,7 @@ To use the feature, admin should set up backing device via::
 before disksize setting. It supports only partition at this moment.
 If admin wants to use incompressible page writeback, they could do via::
 
-	echo huge > /sys/block/zramX/write
+	echo huge > /sys/block/zramX/writeback
 
 To use idle page writeback, first, user need to declare zram pages
 as idle::