Fix README: contributions should be based off 2.3 or higher

Per [the documentation](http://symfony.com/doc/current/contributing/documentation/overview.html#contributing) and [WouterJ's comment here](https://github.com/symfony/symfony-docs/pull/3537#issuecomment-33985191), the base branch should be 2.3, not 2.2.  This commit updates the README to reflect the language found in the documentation.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a