[1.5.x] Fixed #21613 -- Removed a hardcoded input id from the tutorial.

  ``value`` of each radio button is the associated poll choice's ID. The

  ``name`` of each radio button is ``"choice"``. That means, when somebody

  selects one of the radio buttons and submits the form, it'll send the

  POST data ``choice=3``. This is the basic concept of HTML forms.

  POST data ``choice=#`` where # is the ID of the selected choice. This is the

  basic concept of HTML forms.

* We set the form's ``action`` to ``{% url 'polls:vote' poll.id %}``, and we

  set ``method="post"``. Using ``method="post"`` (as opposed to