Removes a mistaken note about the static media development helper.

Turns out that when you write documentation without actually consulting the code
you get things wrong. Who knew?

Thanks to Waldemar Kornewald for catching my mistake.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14299 bcc190cf-cafb-0310-a4f2-bffc1f526a37