webp quality was wrong.

Since it is a getAppValue and not a getSystemValue, the setting cannot be changed like this.
But since it is not yet clear for the rest of the settings which setting in the config is changeable and which is not, there may be another change.
nextcloud/server#38032 (comment)

Signed-off-by: JanisPlayer <54918417+JanisPlayer@users.noreply.github.com>

admin_manual/configuration_files/previews_configuration.rst

@@ -137,4 +137,4 @@ Default WebP quality setting for preview images is '80'. Change this with:
 
 ::
 
-  'webp_quality' => '30',
+  occ config:app:set preview webp_quality --value="30"