Update dashboard.html

Step 2/2 of "Provide a `Logout` button."

These two things, 

1. provide a logout destination URL in the `/_config/bundles/member/member.yaml` file

2. put a logout button in the `/_themes/templates/member/dashboard.html` file

seem to provide the bare minimum to implement a rudimentary logout mechanism--which, IMHO, I think this basic demo ought to have.

I'm new at all this, so there may likely be a better way to do this! (So I wouldn't be the least bit offended if you decided, "no thanks, I'd do it differently.") But however it's done, I think the logout thing is necessary to complete the usefulness of the demo.