time: update documentation for Duration.String regarding the zero value

It was out of date; in 1.7 the format changes to 0s.

Change-Id: I2013a1b0951afc5607828f313641b51c74433257
Reviewed-on: https://go-review.googlesource.com/24421Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/time.go

@@ -454,8 +454,7 @@ const (
 // String returns a string representing the duration in the form "72h3m0.5s".
 // Leading zero units are omitted. As a special case, durations less than one
 // second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
-// that the leading digit is non-zero. The zero duration formats as 0,
-// with no unit.
+// that the leading digit is non-zero. The zero duration formats as 0s.
 func (d Duration) String() string {
 	// Largest time is 2540400h10m10.000000000s
 	var buf [32]byte