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>