[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.
I've tried hard to avoid any `note`, `tip`or `caution` because they introduce breaks into the linear reading of the document and they often include code or command repetitions.

That's why I'd love to remove my original paragraph and your proposed note with the following:

```
Now you can **sync your fork** by executing the following command:

.. code-block:: bash

    $ cd projects/symfony-docs/
    $ git fetch upstream
    $ git checkout 2.3
    $ git merge upstream/2.3

This command will update the ``2.3`` branch, which is the one you used to create
the new branch for your changes. If you have used another base branch, e.g. 
``master``, replace the ``2.3`` value for the appropriate branch name.
```