doc: Remove rcu_preempt_state reference in stallwarn

Consolidation of RCU-bh, RCU-preempt, and RCU-sched into one RCU flavor
to rule them all resulted in the removal of rcu_preempt_state.  However,
stallwarn.txt still mentions rcu_preempt_state.  This commit therefore
Updates stallwarn documentation accordingly.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/stallwarn.txt

@@ -176,9 +176,8 @@ causing stalls, and that the stall was affecting RCU-sched.  This message
 will normally be followed by stack dumps for each CPU.  Please note that
 PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
 the tasks will be indicated by PID, for example, "P3421".  It is even
-possible for a rcu_preempt_state stall to be caused by both CPUs -and-
-tasks, in which case the offending CPUs and tasks will all be called
-out in the list.
+possible for an rcu_state stall to be caused by both CPUs -and- tasks,
+in which case the offending CPUs and tasks will all be called out in the list.
 
 CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
 the RCU core for the past three grace periods.  In contrast, CPU 16's "(0