Currently there is no example for http.Handle in the
documentation. This adds an example.

Change-Id: I66ee9983bea1f5237757e1ef4956eae9a056e963
Reviewed-on: https://go-review.googlesource.com/137715
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>