http2: fix typo

Change-Id: I22b3781a4514b7e8a3778021c465822022b8bdc2
Reviewed-on: https://go-review.googlesource.com/34499
Reviewed-by: Tom Bergan <tombergan@google.com>
diff --git a/http2/server.go b/http2/server.go
index 4d7ed60..0431ab0 100644
--- a/http2/server.go
+++ b/http2/server.go
@@ -944,7 +944,7 @@
 				panic(fmt.Sprintf("internal error: attempt to send frame on a half-closed-local stream: %v", wr))
 			}
 		case stateClosed:
-			panic(fmt.Sprintf("internal error: attempt to send frame a closed stream: %v", wr))
+			panic(fmt.Sprintf("internal error: attempt to send frame on a closed stream: %v", wr))
 		}
 	}
 	if wpp, ok := wr.write.(*writePushPromise); ok {