net/http/httptrace: fix bad tracing example

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>

src/net/http/httptrace/example_test.go

@@ -6,6 +6,7 @@ package httptrace_test
 
 import (
 	"fmt"
+	"log"
 	"net/http"
 	"net/http/httptrace"
 )
@@ -21,5 +22,8 @@ func Example() {
 		},
 	}
 	req = req.WithContext(httptrace.WithClientTrace(req.Context(), trace))
-	http.DefaultClient.Do(req)
+	_, err := http.DefaultTransport.RoundTrip(req)
+	if err != nil {
+		log.Fatal(err)
+	}
 }