net/http, cmd/compile: minor vet fixes

Updates #11041

Change-Id: Ia0151723e3bc0d163cc687a02bfc5e0285d95ffa
Reviewed-on: https://go-review.googlesource.com/27810
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/scanner_test.go

@@ -79,7 +79,7 @@ func TestTokens(t *testing.T) {
 				continue
 			}
 			if got.prec != want.prec {
-				t.Errorf("got prec = %s; want %s", got.prec, want.prec)
+				t.Errorf("got prec = %d; want %d", got.prec, want.prec)
 				continue
 			}
 			nlsemi = want.tok == _IncOp

src/net/http/httptrace/example_test.go

@@ -10,7 +10,7 @@ import (
 	"net/http/httptrace"
 )
 
-func ExampleTrace() {
+func Example() {
 	req, _ := http.NewRequest("GET", "http://example.com", nil)
 	trace := &httptrace.ClientTrace{
 		GotConn: func(connInfo httptrace.GotConnInfo) {