minor #9610 Use "children" property instead of "child" (liviubalan)

This PR was submitted for the 3.4 branch but it was merged into the 2.7 branch instead (closes #9610).

Discussion
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Use "children" property instead of "child"

Fix error: Neither the property "child" nor one of the methods "child()", "getchild()"/"ischild()"/"haschild()" or "__call()" exist and have public access in class "Symfony\Component\Form\FormView".

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Commits
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26349df Use "children" property instead of "child"

Author: javiereguiluz

form/form_customization.rst

@@ -337,7 +337,7 @@ You can also apply a form theme to a specific child of your form:
 
 .. code-block:: html+twig
 
-    {% form_theme form.child 'form/fields.html.twig' %}
+    {% form_theme form.children 'form/fields.html.twig' %}
 
 This is useful when you want to have a custom theme for a nested form that's
 different than the one of your main form. Just specify both your themes:
@@ -346,7 +346,7 @@ different than the one of your main form. Just specify both your themes:
 
     {% form_theme form 'form/fields.html.twig' %}
 
-    {% form_theme form.child 'form/fields_child.html.twig' %}
+    {% form_theme form.children 'form/fields_child.html.twig' %}
 
 Form Theming in PHP
 -------------------