io/i2c: add an example for Open
io/i2c: add an example for Open

Change-Id: Ie9ad7a09b91f53372107dae19cb01dfd09978a94
Reviewed-on: https://go-review.googlesource.com/22517
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>
Reviewed-by: Johan Euphrosine <aeff70c2aef2cec1cd2754756b5d3e6402001c31@google.com>

golang/exp,golang/exp,golang/exp