Fix webhook sample in C# class library article

Customer left this note in LiveFyre on this article (https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-class-library)

> I think there's a bug in the example for "HTTP and webhooks". The [HttpTrigger] is missing a `methods` argument and because of this the example returns "404 Not Found". More details here: https://stackoverflow.com/questions/44500099/why-am-i-getting-a-404-when-i-run-my-azure-function-locally

> A fix might look like:
> [HttpTrigger(AuthorizationLevel.Anonymous, "GET", WebHookType = "genericJson")]

We could just use the same code that's in the template: https://github.com/Azure/azure-webjobs-sdk-templates/blob/dev/Functions.Templates/Templates/GenericWebHook-CSharp/GenericWebHookCSharp.cs#L24

Read the “HTTP and webhooks” example in the Azure Functions class library article, then compare it with Functions.Templates/Templates/GenericWebHook-CSharp/GenericWebHookCSharp.cs at the referenced line. Done means the article’s sample includes the required trigger argument and no longer produces the reported 404 behavior.

Written by the indexing model from the issue text.