doc: Update whatisRCU.rst

The kfree_rcu() macro is deprecated.  Rename it to its new
kfree_rcu_mightsleep() name in this documentation.

Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>