Improve `add-new-pipeline` docs a bit
# What does this PR do?

This PR slightly improve syntaxs in code snippets in the `add-new-pipeline.rst` file.


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.
