time: move the explanation of u/micro to the ParseDuration example

Fix a few missing capitalizations in drive-by.

Change-Id: I7353c12f3ccddefc0f26a98590caf9e446129558
Reviewed-on: https://go-review.googlesource.com/c/163918


Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>