Update HubProtocol.md (#12509)
Update HubProtocol.md (#12509)

* Update HubProtocol.md

Updated the code sample in the protocol doc to reflect the actual programming API

* Update src/SignalR/docs/specs/HubProtocol.md

Co-Authored-By: Brennan <7d5944d843db024d954983f0eccaa5b8890be4de@microsoft.com>

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