time: fix typo in ExampleParseDuration

time: fix typo in ExampleParseDuration

A typo without consequences, but confusing nonetheless. The last
line prints micro2 and then micro, instead of micro2 twice.
One-character fix.

Fixes #65666

Change-Id: I61d636382a2223d53eac58d6ddbcc7c15b4efc85
Reviewed-on: https://go-review.googlesource.com/c/go/+/563275
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>