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://codereview.appspot.com/7279045

Committer: Russ Cox <rsc@golang.org>