[Docs] Minor fixes: Typos, Formatting, Links, Sample code

* The API link for ``ParamConverterInterface`` did not work so I've removed it
* In the overall annotation code example I've changed the template name ``post.html.twig`` to ``show.html.twig`` so the following statement in the doc becomes true: *As the `showAction` method follows some conventions, you can omit some annotations:*