[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.
On https://github.com/symfony/symfony-docs, the number of contributors is 721. Do you know where the figure on symfony.com is coming from?