Documentation: RCU: Retitle toctree index

toctree index title ("RCU concepts") is rather a misnomer: RCU concepts
is already described in rcu.rst, whereas the toctree suggests that
the docs themes about handbook on RCU subsystem instead.

Edit the title to reflect the fact.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: bagasme

Documentation/RCU/index.rst

@@ -1,9 +1,9 @@
 .. SPDX-License-Identifier: GPL-2.0
 
-.. _rcu_concepts:
+.. _rcu_handbook:
 
 ============
-RCU concepts
+RCU Handbook
 ============
 
 .. toctree::