doc: Make listing in RCU perf/scale requirements use rcu_assign_pointer()

The code listing under this section has a quick quiz that says line
19 uses rcu_access_pointer, but the code listing itself instead uses
rcu_dereference().  This commit therefore makes the code listing match
the quick quiz.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/Design/Requirements/Requirements.html

@@ -1596,7 +1596,7 @@ used in place of <tt>synchronize_rcu()</tt> as follows:
 16   struct foo *p;
 17
 18   spin_lock(&amp;gp_lock);
-19   p = rcu_dereference(gp);
+19   p = rcu_access_pointer(gp);
 20   if (!p) {
 21     spin_unlock(&amp;gp_lock);
 22     return false;