Doc: Fix spelling in RCU/rculist_nulls.txt.

Trival spelling fixes in RCU/rculist_nulls.txt.
Reviewed-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NJesper Dangaard Brouer <hawk@comx.dk>
Tested-by: NJarek Poplawski <jarkao2@gmail.com;->
Signed-off-by: NDavid S. Miller <davem@davemloft.net>