Improvements for instructions on opening a pull request

## 1. Merge to main or dev?
In this paragraph 'Raising a pull request': https://carpentries-incubator.github.io/python-intermediate-development/41-code-review/index.html#raising-a-pull-request . In point 4 we say: Select the base and the compare branch - main and feature-std-dev, respectively. In the screenshots the merge is to the `dev` branch however. I would suggest to merge into `dev` here.

## 2. Adapt for forked repo
The screenshots and instructions show the situation in the original repo. But participants work in a forked repo. The screenshots should reflect this. And make it very clear that they should not open a PR to the forked repo, which is the default in GitHub.