net/http: fix nit in test

Change-Id: I8c647e709d93a76636e04375609fceadf3754aa1
Reviewed-on: https://go-review.googlesource.com/18954Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/response_test.go

@@ -822,7 +822,7 @@ func TestReadResponseErrors(t *testing.T) {
 		if err := matchErr(rerr, tt.wantErr); err != nil {
 			name := tt.name
 			if name == "" {
-				name = fmt.Sprintf("%i. input %q", i, tt.in)
+				name = fmt.Sprintf("%d. input %q", i, tt.in)
 			}
 			t.Errorf("%s: %v", name, err)
 		}