encoding/xml: fix format in test message

Found by go vet.

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews
https://golang.org/cl/100510044

src/pkg/encoding/xml/marshal_test.go

@@ -1260,7 +1260,7 @@ func TestDecodeEncode(t *testing.T) {
 	for tok, err := dec.Token(); err == nil; tok, err = dec.Token() {
 		err = enc.EncodeToken(tok)
 		if err != nil {
-			t.Fatalf("enc.EncodeToken: Unable to encode token (%#v), %d", tok, err)
+			t.Fatalf("enc.EncodeToken: Unable to encode token (%#v), %v", tok, err)
 		}
 	}
 }