[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.
@xabbuh the github UI counts the number of github accounts. symfony.com counts the number of distinct email addresses. So there is 2 sources of differences:

- people with several email addresses associated to their github account
- people contributing with an email address not associated to their github account

@javiereguiluz why would it be outdated ? 850 is still more than 700 :smile:. The only way it could be outdated is when the number of contributors become lower than 700, which is not possible