[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.
Indeed, that's a nice proposition. I changed it here and also replaced the master branch in the sync example below with 2.3 too.