Fix comment in HPA's scale event replicaChange

The field replicaChange in timestampedScaleEvent was wrongly described
as either positive or negative depending on the scale direction. In
fact the change is set as unsigned, positive or 0 even for downscales.

pkg/controller/podautoscaler/horizontal.go

@@ -62,7 +62,7 @@ type timestampedRecommendation struct {
 }
 
 type timestampedScaleEvent struct {
-	replicaChange int32 // positive for scaleUp, negative for scaleDown
+	replicaChange int32 // absolute value, non-negative
 	timestamp     time.Time
 	outdated      bool
 }