net/http: add Handle example
net/http: add Handle example

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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,josharian/go,josharian/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,josharian/go,CAFxX/go,golang/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,josharian/go,akutz/go,golang/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,akutz/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,josharian/go,golang/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go