Documentation: RCU: Reduce toctree depth

toctree index for RCU currently has maximum depth of 3. Since no docs
currently use more than 3 section heading levels, this effectively spills
the entire docs hierarchy.

Tidy up by reducing toctree depth to 2 (only showing docs title and
second-level sections).

Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/index.rst

···
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 .. toctree::
-   :maxdepth: 3
+   :maxdepth: 2
 
    checklist
    lockdep