Docs/RCU/rculist_nulls: Fix trivial coding style

Lookup example of non-hlist_nulls management is missing a semicolon, and
having inconsistent indentation (one line is using single space
indentation while others are using two spaces indentation).  Fix the
trivial issues.

Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>