docs(guide/controller): only show best practice controller creation

If it is not recommended to use a global function to create controllers,
why should it be shown as possible in the documentation?

One of the most common complaints about AngularJS is that it doesn't enforce
any convention. This is intentional and I generally like this.
However if we can avoid outright bad implementations in examples I believe
we should.

Closes #8011
docs(guide/controller): tweak initial example