Update dashboard.txt

[flxsosa]

Updating dashboard.txt to instruct users to also set secret_key as a config variable if they want to enable the dashboard. Before this commit, secret_key is not mentioned as a needed variable, leading to a PsiturkException when you enable the dashboard without secret_key.

Please review whether you can reference the secret_key variable through :ref:settings-secret-key. I was not sure if this was available as a reference but was following the syntax of :ref:settings-login-*

[deargle]

merged via 956ef9f

[deargle]

Thanks! lol there wasn't a secret_key section in the settings.rst at all