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>