Fixed link to SignalR transport protocol spec (#55)

Author: sanjaysingh

Diff for: samples/ChatRoom/README.md

@@ -67,7 +67,7 @@ Let's look at the key changes:
 
 Other than these changes, everything else remains the same, you can still use the hub interface you're already familiar with to write business logic.
 
-> Under the hood, an endpoint `/chat/negotiate` is exposed for negotiation by Azure SignalR Service SDK. It will return a special negotiation response when clients try to connect and redirect clients to service endpoint from the connection string. Read more details about the redirection at SignalR's [Negotitation Protocol](https://github.com/aspnet/SignalR/blob/dev/specs/TransportProtocols.md#post-endpoint-basenegotiate-request).
+> Under the hood, an endpoint `/chat/negotiate` is exposed for negotiation by Azure SignalR Service SDK. It will return a special negotiation response when clients try to connect and redirect clients to service endpoint from the connection string. Read more details about the redirection at SignalR's [Negotitation Protocol](https://github.com/aspnet/SignalR/blob/master/specs/TransportProtocols.md#post-endpoint-basenegotiate-request).
 
 
 Now set the connection string in the [Secret Manager](https://docs.microsoft.com/en-us/aspnet/core/security/app-secrets?view=aspnetcore-2.1&tabs=visual-studio#secret-manager) tool for .NET Core, and run this app.