Fix article guidance query

Fixes missing comma:

pyspark.sql.utils.ParseException:
no viable alternative at input
...
), t_returned AS (
  -- Subjects who published a second article via Article Guidance
  -- strictly after their first and within 30 days of it.
  SELECT DISTINCT
    IF(t_first_article.assigned = 'control', 'control', 'treatment') AS variation,
    t_first_article.subject_id
    COUNT(return_action.meta.dt) AS returns
---------^^^
  FROM t_first_article
...

Bug: T422146