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

Thanks Bruno Brouard for the report.

Backport of cc2d960274 from master

docs/intro/tutorial04.txt

@@ -33,7 +33,8 @@ A quick rundown:
   ``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