Update UPGRADE-2.3.md to account for https://github.com/symfony/symfony/pull/9388

Added documentation for how to correctly pre-fill a form using the form's `data` option.
The "old" and also wrong way of doing it broke with Symfony 2.3.7 (with https://github.com/symfony/symfony/pull/9388 to be precise) and this short documentation should help others to fix the problem and do it right.