docs: improve CPU time query

Fix the issue of the missing WHEN by instead switching to IIF which
makes the query clearer.

Fixes: #655
Change-Id: Id8c42e77033c1c01abac33f807932d1926389243

docs/analysis/common-queries.md

@@ -52,8 +52,8 @@ For example to get the time spent "runnable" and in "uninterruptible sleep":
 SELECT
   slice_id,
   slice_name,
-  SUM(CASE state = 'R' THEN dur ELSE 0 END) AS runnable_time,
-  SUM(CASE state = 'D' THEN dur ELSE 0 END) AS uninterruptible_time
+  SUM(IIF(state = 'R', dur, 0)) AS runnable_time,
+  SUM(IIF(state = 'D', dur, 0)) AS uninterruptible_time
 FROM slice_thread_state_breakdown
 GROUP BY slice_id;
 ```