Doc: Fix spelling in RCU/rculist_nulls.txt.

Doc: Fix spelling in RCU/rculist_nulls.txt.

Trival spelling fixes in RCU/rculist_nulls.txt.

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Tested-by: Jarek Poplawski <jarkao2@gmail.com;->
Signed-off-by: David S. Miller <davem@davemloft.net>