Type the module translator example against the interface

[boo-code]

The "Other classes" example in modules/creation/module-translation/new-system.md is what
PrestaShop/PrestaShop#34138 reports. It is broken three ways:

• public function __construct(Translator $translator) names a concrete class. The container decides
  which translator a module receives and the debug environment decorates it, so a class following the
  example works with debug off and throws with it on.
• Translator is unqualified, so inside a module namespace it resolves to a class that does not exist.
• $this->translator = $translator is missing its semicolon, so the snippet does not compile.

The change types the parameter as PrestaShopBundle\Translation\TranslatorInterface, adds the use
line the snippet needs, fixes the semicolon, and says in one sentence why the interface rather than a
concrete class.

Paired with a core branch (fix/translator-contract-is-the-interface-34138, base develop) that makes
that interface declare everything the core calls on a translator, since it previously did not declare
setLocale() or getCatalogue().