Minor edits

Added a couple citations for the recent CyberShake study and for the
Galactic Plane workflows, and made a few minor edits to the text.

I also changed the description of the recent CyberShake study, which
did look at 286 sites, but required 2 simulation codes * 2 velocity
models * 286 sites * 2 workflows (sgt and post-processing) = 2288
workflows, rather than 286 workflows.