rcu: Fix typo in rcu_get_gp_kthreads_prio() header comment

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

kernel/rcu/tree.c

@@ -181,7 +181,7 @@ module_param(gp_init_delay, int, 0444);
 static int gp_cleanup_delay;
 module_param(gp_cleanup_delay, int, 0444);
 
-/* Retreive RCU kthreads priority for rcutorture */
+/* Retrieve RCU kthreads priority for rcutorture */
 int rcu_get_gp_kthreads_prio(void)
 {
 	return kthread_prio;