Documentation: add watermark_scale_factor to the list of vm systcl file

Commit 795ae7a0 ("mm: scale kswapd watermarks in proportion to
memory") properly added the description of the new knob to
Documentation/sysctl/vm.txt, but forgot to add it to the list of files
in /proc/sys/vm. Let's fix that.
Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sysctl/vm.txt

@@ -61,6 +61,7 @@ Currently, these files are in /proc/sys/vm:
 - swappiness
 - user_reserve_kbytes
 - vfs_cache_pressure
+- watermark_scale_factor
 - zone_reclaim_mode
 
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