Update rubric to recommend maximizing the browser window (instead of "fullscreen")

As discussed[1], change the rubric to suggest maximizing the browser window,
instead of taking it into fullscreen. The about page clarifies that leaves
system UI visible (i.e. the macOS use of "maximize").

[1] https://docs.google.com/document/d/10dE1HHht5j1F4woUu8Z9pClksWU1gO7GUPQU-Rn_ah8/#heading=h.lmrf9obg3lrc