container/heap: avoid and/or ambiguity in documentation

(per suggestion by Doug McIlroy)

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/50580046

src/pkg/container/heap/heap.go

@@ -22,7 +22,7 @@ import "sort"
 // min-heap with the following invariants (established after
 // Init has been called or if the data is empty or sorted):
 //
-//	!h.Less(j, i) for 0 <= i < h.Len() and j = 2*i+1 or 2*i+2 and j < h.Len()
+//	!h.Less(j, i) for 0 <= i < h.Len() and 2*i+1 <= j <= 2*i+2 and j < h.Len()
 //
 // Note that Push and Pop in this interface are for package heap's
 // implementation to call.  To add and remove things from the heap,