[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.
In http://symfony.com/contributors/doc you can se that there are 794 people identified as documentation contributors. Don't you think it's better to say *more than 800*? Otherwise, this figure will be out-dated pretty soon.