Replace <> icon with a recognizable Markdown icon.

[carlsmith]

I used the Studio quite a few time before I realized how to edit a Markdown file. I wanted to edit them, but didn't see how, and just assumed they were placeholders that would be filled out with some kind of docs in the near future.

The current button is too easy to miss or misinterpret. The Studio seems to be using the Octicons icon set, which includes a Markdown icon in this style:

Maybe that icon would be more obvious.