docs/scheduler: Change unit of cpu_time and rq_time to nanoseconds

In the current implementation, cpu_time and rq_time should be
in nanoseconds, so this document needs to be modified.
Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/scheduler/sched-stats.rst

@@ -56,9 +56,9 @@ Next two are try_to_wake_up() statistics:
 
 Next three are statistics describing scheduling latency:
 
-     7) sum of all time spent running by tasks on this processor (in jiffies)
+     7) sum of all time spent running by tasks on this processor (in nanoseconds)
      8) sum of all time spent waiting to run by tasks on this processor (in
-        jiffies)
+        nanoseconds)
      9) # of timeslices run on this cpu