doc/kprobes: Update obsolete RCU update functions

The RCU flavors have been consolidated, so this commit replaces mentions
of the now-obsolete synchronize_sched() function with synchronize_rcu().
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <linux-doc@vger.kernel.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

Documentation/kprobes.txt

@@ -243,10 +243,10 @@ Optimization
 ^^^^^^^^^^^^
 
 The Kprobe-optimizer doesn't insert the jump instruction immediately;
-rather, it calls synchronize_sched() for safety first, because it's
+rather, it calls synchronize_rcu() for safety first, because it's
 possible for a CPU to be interrupted in the middle of executing the
-optimized region [3]_.  As you know, synchronize_sched() can ensure
-that all interruptions that were active when synchronize_sched()
+optimized region [3]_.  As you know, synchronize_rcu() can ensure
+that all interruptions that were active when synchronize_rcu()
 was called are done, but only if CONFIG_PREEMPT=n.  So, this version
 of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.