[feature] Add blank line after namespace fixer

well, it is the standard we use everywhere in the Symfony codebase. It is not written explicitly in the doc (they only have the rule about multiline arrays), but this page is known to be incomplete. This is why they started the page by saying ``To make a long story short, here is the golden rule: Imitate the existing Symfony2 code.`` so that we cannot tell them all the time that they are missing stuff. We should update the page as well.