chore: update docs with correct examples of the AnyCable.broadcast usage

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Summary

Update docs with correct examples of the AnyCable.broadcast usage.
Current examples tells that Hash can be passed as 2nd parameter. But it is not. It works with ActionCable but not with AnyCable

ActionCable.server.broadcast("echo_channel", {text: "hoi"}) # works
AnyCable.broadcast("echo_channel", {text: "hoi"}) # doesn't work
AnyCable.broadcast("echo_channel", JSON.dump({text: "hoi"})) # works