time: simplify Duration.String example

The existing example is needlessly complex.
You have to know that t.Sub returns a Duration
and also have to mentally subtract the two times
to understand what duration should be printed.

Rewrite to focus on just the Duration.String operation.

Change-Id: I00765b6019c07a6ff03022625b556c2b9ba87c09
Reviewed-on: https://go-review.googlesource.com/c/go/+/234893
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/time/example_test.go b/src/time/example_test.go
index 15811a6..0f9b874 100644
--- a/src/time/example_test.go
+++ b/src/time/example_test.go
@@ -50,10 +50,11 @@
 }
 
 func ExampleDuration_String() {
-	t1 := time.Date(2016, time.August, 15, 0, 0, 0, 0, time.UTC)
-	t2 := time.Date(2017, time.February, 16, 0, 0, 0, 0, time.UTC)
-	fmt.Println(t2.Sub(t1).String())
-	// Output: 4440h0m0s
+	fmt.Println(1*time.Hour + 2*time.Minute + 300*time.Millisecond)
+	fmt.Println(300*time.Millisecond)
+	// Output:
+	// 1h2m0.3s
+	// 300ms
 }
 
 func ExampleDuration_Truncate() {