Link to the diagnose documentation from the demo error message (#512)
Link to the diagnose documentation from the demo error message (#512)

> Error: Unable to start the AppSignal agent and send data to AppSignal.com.
> Please use the diagnose command (https://docs.appsignal.com/ruby/command-line/diagnose.html) to debug your configuration:
>
>       bundle exec appsignal diagnose --environment=production
>
appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby