websocket: fix printf(var) mistake detected by latest printf checker

For golang/go#69267.

Change-Id: Ica6d123312495966ad6d222d67944fd602216853
Reviewed-on: https://go-review.googlesource.com/c/net/+/610799
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

websocket/hybi_test.go

@@ -163,7 +163,7 @@ Sec-WebSocket-Protocol: chat
 	}
 	for k, v := range expectedHeader {
 		if req.Header.Get(k) != v {
-			t.Errorf(fmt.Sprintf("%s expected %q but got %q", k, v, req.Header.Get(k)))
+			t.Errorf("%s expected %q but got %q", k, v, req.Header.Get(k))
 		}
 	}
 }