documentation: Update sysfs path for rcu_cpu_stall_suppress

Commit 6bfc09e2 (rcu: Provide RCU CPU stall warnings for tiny RCU)
moved the rcu_cpu_stall_suppress module parameter from rcutree.c to
rcupdate.c, but failed to update Documentation/RCU/stallwarn.txt.  This
commit therefore repairs this omission.
Reported-by: NKirill Tkhai <tkhai@yandex.ru>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Documentation/RCU/stallwarn.txt

@@ -24,7 +24,7 @@ CONFIG_RCU_CPU_STALL_TIMEOUT
 	timing of the next warning for the current stall.
 
 	Stall-warning messages may be enabled and disabled completely via
-	/sys/module/rcutree/parameters/rcu_cpu_stall_suppress.
+	/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
 
 CONFIG_RCU_CPU_STALL_VERBOSE