container/heap: fix comment typo in example test

This updates a bad reference to a method name in the example priority queue test.

The error was introduced in the example refactoring in rev. 2ea8f07b2ffe.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7279045

src/pkg/container/heap/example_pq_test.go

@@ -14,7 +14,7 @@ import (
 type Item struct {
 	value    string // The value of the item; arbitrary.
 	priority int    // The priority of the item in the queue.
-	// The index is needed by changePriority and is maintained by the heap.Interface methods.
+	// The index is needed by update and is maintained by the heap.Interface methods.
 	index int // The index of the item in the heap.
 }