docs: rcu: Increase toctree to 3

These documents are long and have various sections. Provide a good
toc nesting level.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/index.rst

@@ -5,7 +5,7 @@ RCU concepts
 ============
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 3
 
    rcu
    listRCU