minor #10036 fix code block indentation (xabbuh)

This PR was merged into the 4.0 branch.

Discussion
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fix code block indentation

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Commits
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ef4e18a fix code block indentation

Author: xabbuh

form/form_themes.rst

@@ -26,16 +26,16 @@ do this, create a new template file that will store the new markup:
 
     .. code-block:: html+twig
 
-    {# templates/form/fields.html.twig #}
-    {% block form_row %}
-    {% spaceless %}
-        <div class="form_row">
-            {{ form_label(form) }}
-            {{ form_errors(form) }}
-            {{ form_widget(form) }}
-        </div>
-    {% endspaceless %}
-    {% endblock form_row %}
+        {# templates/form/fields.html.twig #}
+        {% block form_row %}
+        {% spaceless %}
+            <div class="form_row">
+                {{ form_label(form) }}
+                {{ form_errors(form) }}
+                {{ form_widget(form) }}
+            </div>
+        {% endspaceless %}
+        {% endblock form_row %}
 
 The ``form_row`` form fragment is used when rendering most fields via the
 ``form_row()`` function. To tell the Form component to use your new ``form_row``