doc: Fixup definition of rcupdate.rcu_task_stall_timeout

A positive value of rcupdate.rcu_task_stall_timeout is an interval
in seconds rather than jiffies.
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/stallwarn.txt

@@ -153,7 +153,7 @@ rcupdate.rcu_task_stall_timeout
 	This boot/sysfs parameter controls the RCU-tasks stall warning
 	interval.  A value of zero or less suppresses RCU-tasks stall
 	warnings.  A positive value sets the stall-warning interval
-	in jiffies.  An RCU-tasks stall warning starts with the line:
+	in seconds.  An RCU-tasks stall warning starts with the line:
 
 		INFO: rcu_tasks detected stalls on tasks: