Updated link to correct twig docs

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1-
| Fixed tickets | -

Creating extensions had a big change in Twig 1.12. That means they completely rewritten that chapter. We should update the docs to link to the correct article (Sf2.1- should get a link to the old article, while 2.2+ should use the current article).

**Please note that this should not be merged in 2.2, as that comes with Twig 1.12**