[Cookbook][Assetic] Fix "javascripts" tag name typo

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Maybe the following note *This will most commonly live in the ``javascripts`` block...* is a bit confusing because the example does not show this Twig block and it has the same name as the Twig tag... The note could either be removed or I could add the `javascripts` Twig block to clarify?