Explicitly call out set/get in Usage example.

I've heard from a few students that the docs for connect-flash didn't call out the getter/setter as much as  they ought to, so I added a couple of comments to the set and get lines in the Usage section of README.md to make it clearer for future noders. I also added syntax highlighting because, well, I like syntax highlighting.

If the comments make sense, I'd appreciate a pull.