Mention that students have to change the 'base fork' when making the pull request

[oliverjam]

A few of us have accidentally opened pull requests against Jen's original repo rather than our own fork. It might be worth mentioning in the instructions that we need to change the base fork in the dropdown to ensure the pull request opens against our own fork.

[NataliaLKB]

Haha yes lots of PRs have been opened here because of this. I always figure people will figure it out, and it will help them understand github better.

Feel free to make a PR to clarify.