fix golint issues in core/timex (#517)

core/timex/repr.go

@@ -5,6 +5,7 @@ import (
 	"time"
 )
 
+// ReprOfDuration returns the string representation of given duration in ms.
 func ReprOfDuration(duration time.Duration) string {
 	return fmt.Sprintf("%.1fms", float32(duration)/float32(time.Millisecond))
 }