Doc: Fix wrong API example usage of call_rcu().

At some point the API of call_rcu() changed from three parameters
to two parameters, correct the documentation.

One confusing thing in RCU/listRCU.txt, which is NOT fixed in this patch,
is that no reason or explaination is given for using call_rcu() instead of
the normal synchronize_rcu() call.

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

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>