container/heap: avoid memory leak in example

Set element in slice to nil avoiding memory leak.

Change-Id: I9dbef9a0466407011e326725d3a0b681cd815389
GitHub-Last-Rev: 1bae5d375876a7f146eb04c2a5ba88d079264eb3
GitHub-Pull-Request: golang/go#30386
Reviewed-on: https://go-review.googlesource.com/c/163601
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/container/heap/example_pq_test.go

@@ -45,6 +45,7 @@ func (pq *PriorityQueue) Pop() interface{} {
 	old := *pq
 	n := len(old)
 	item := old[n-1]
+	old[n-1] = nil  // avoid memory leak
 	item.index = -1 // for safety
 	*pq = old[0 : n-1]
 	return item