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>