[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.
If symfony.com counts unique e-mail addresses, the GitHub count seems more reasonable to me. For example, I used at least two different e-mail addresses in commit messages and I see no reason why this should be counted for two contributors.

Admittedly, this may look weird if people have at the contributors on symfony.com after reading this sentence. However, when you read this and then see the figure on GitHub, this seems even more weird. As @stof said, "more than 700" is true for both figures. :)