http2: fix a nit

Change-Id: I3798117511b4c9d9033a3b5dbefbae34d7bec639
Reviewed-on: https://go-review.googlesource.com/19244
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/http2/transport_test.go b/http2/transport_test.go
index 827bfdb..e9b89f7 100644
--- a/http2/transport_test.go
+++ b/http2/transport_test.go
@@ -387,7 +387,7 @@
 		defer res.Body.Close()
 		ri := <-gotc
 		if ri.err != nil {
-			t.Errorf("%#d: read error: %v", i, ri.err)
+			t.Errorf("#%d: read error: %v", i, ri.err)
 			continue
 		}
 		if got := string(ri.slurp); got != tt.body {