tools/memory-model/Documentation: Fix SRCU section in explanation.txt

The SRCU read-side critical sections describes the difference between
srcu_down_read()/srcu_up_read() and srcu_read_lock()/srcu_read_unlock()
in a way that a last pair must occur on the same CPU.

This is not true, the srcu_read_unlock() can happen on any CPU, but it
must be performed by the same task that invoked srcu_read_lock().

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>