Tracing happens at the http.Trace level. Fix the example to demostrate
tracing in the lifecycle of a RoundTrip.

Updates #17152.

Change-Id: Ic7d7bcc550176189206185482e8962dbf1504ff1
Reviewed-on: https://go-review.googlesource.com/29431Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>