fix stack fill example (#263)

Author: thomasp85

doc/syntax/layer/type/area.qmd

@@ -67,12 +67,12 @@ DRAW area
   SETTING position => 'identity', opacity => 0.5
 ```
 
-When `position => 'stack_fill'` we're plotting stacked proportions. These only make sense if every series is measured in the same absolute unit. (Wind and temperature have different units and the temperature is not absolute.)
+Whith the default `position => 'stack'` we can normalise the total so that each stack totals to the same value. These only make sense if every series is measured in the same absolute unit. (Wind and temperature have different units and the temperature is not absolute.)
 
 ```{ggsql}
 VISUALISE Date AS x, Value AS y, Series AS colour FROM long_airquality
 DRAW area 
-  SETTING position => 'fill'
+  SETTING total => 100
 ```
 
 An alternative is to center the stacks to create a steamgraph