minor #8095 Add form_theme in error customization (rubenrubiob)

This PR was submitted for the 3.3 branch but it was merged into the 2.7 branch instead (closes #8095).

Discussion
----------

Add form_theme in error customization

User form_theme __self in error examples as in other examples of the page.

Commits
-------

d5c5ff1 Add form_theme in error customization

Author: xabbuh

form/form_customization.rst

@@ -866,6 +866,8 @@ and customize the ``form_errors`` fragment.
 
     .. code-block:: html+twig
 
+        {% form_theme form _self %}
+
         {# form_errors.html.twig #}
         {% block form_errors %}
             {% spaceless %}
@@ -926,6 +928,8 @@ fields (e.g. a whole form), and not just an individual field.
 
     .. code-block:: html+twig
 
+        {% form_theme form _self %}
+
         {# form_errors.html.twig #}
         {% block form_errors %}
             {% spaceless %}