Fix code snippet in readme

[ueman]

When copying the UserAgentClient code snippet, the Dart linter immediately complains that the send method is missing the @override annotation. This change fixes that.

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[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:http	pkgs/http/README.md

Changes to files need to be accounted for in their respective changelogs.

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Coverage ✔️

File	Coverage

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/http/example/main.dart

[mosuem]

Seems like the CI is red here - maybe #1432 will fix that. But this is good to go.