web_socket_channel: fix docs in readme and in class

[kevmoo]

Some broken links and outdated bits

Fixes #1897

[github-actions]

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pkgs/web_socket_channel/lib/html.dart	💔 Not covered

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Unused Dependencies ✔️

Package	Status
web_socket_channel	✔️ All dependencies utilized correctly.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
web_socket_channel	None	3.0.3	3.0.4-wip	3.0.4-wip	✔️

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pkgs/cupertino_http/example/example.dart
pkgs/http/example/main.dart
pkgs/http_multi_server/test/cert.dart

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