container/heap: fix package doc comment about ordering.

R=gri, rsc
CC=golang-dev
https://golang.org/cl/7280044

src/pkg/container/heap/heap.go

@@ -4,7 +4,7 @@
 
 // Package heap provides heap operations for any type that implements
 // heap.Interface. A heap is a tree with the property that each node is the
-// highest-valued node in its subtree.
+// minimum-valued node in its subtree.
 //
 // A heap is a common way to implement a priority queue. To build a priority
 // queue, implement the Heap interface with the (negative) priority as the