Clarify "Modify Project" in `first-program.md`

[k4y3ff]

At the Austin workshop, one or two folks missed the "Modify Project" section of first-program.md and thought that they should be editing the original core.clj file in some way.

Writing this issue while I'm at the workshop, so I'm being brief, but we should brainstorm a way to make that section clearer and/or more prominent.