Docs/RCU/rculist_nulls: Specify type of the object in examples

The type of 'obj' in example code of rculist_nulls.rst is implicit.
Provide the specific type of it before the example code.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/rcu/43943609-f80c-4b6a-9844-994eef800757@paulmck-laptop/


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>