minor #5357 [Form] Replace deprecated form_enctype by form_start (JMLamodiere)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Replace deprecated form_enctype by form_start

Deprecated since 2.3. See [form chapter](http://symfony.com/doc/2.3/book/forms.html#rendering-the-form)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #4603

Commits
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f771d15 Replace deprecated form_enctype by form_start

Author: wouterj

components/form/introduction.rst

@@ -480,11 +480,11 @@ helper functions:
 
 .. code-block:: html+jinja
 
-    <form action="#" method="post" {{ form_enctype(form) }}>
+    {{ form_start(form) }}
         {{ form_widget(form) }}
 
         <input type="submit" />
-    </form>
+    {{ form_end(form) }}
 
 .. image:: /images/book/form-simple.png
     :align: center