sync/atomic: use package prefix in examples

sync/atomic: use package prefix in examples

Previously these examples declared "var v Value" but any caller would
need to write "var v atomic.Value", so we should use the external
package declaration form to avoid confusion about where Value comes
from.

Change-Id: Ic0b1a05fb6b700da61cfc8efca594c49a9bedb69
Reviewed-on: https://go-review.googlesource.com/107975
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>