tweaks to the contribution chapter
minor #4245 [Contributing] tweaks to the contribution chapter (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Contributing] tweaks to the contribution chapter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Some tweaks for the changes made in #4223.

@javiereguiluz Sorry,  didn't find the time to review it in the first run.

Commits
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f4b0b8e tweaks to the contribution chapter