rcu: Fix obsolete priority-boosting comment

Tasks are no longer migrated to the root rcu_node, so there is no
longer any need for a boost kthread for the root rcu_node, and there no
longer is such a kthread.  This commit therefore fixes the comment in
rcu_boost_kthread()'s header to reflect this new reality.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1061,8 +1061,7 @@ static int rcu_boost(struct rcu_node *rnp)
 }
 
 /*
- * Priority-boosting kthread.  One per leaf rcu_node and one for the
- * root rcu_node.
+ * Priority-boosting kthread, one per leaf rcu_node.
  */
 static int rcu_boost_kthread(void *arg)
 {