minor #10372 [Forms] Update Bootstrap 4's documentation links to 4.1 (royklutman)

This PR was squashed before being merged into the 3.4 branch (closes #10372).

Discussion
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[Forms] Update Bootstrap 4's documentation links to 4.1

Commits
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c3731ba [Forms] Update Bootstrap 4's documentation links to 4.1

Author: javiereguiluz

form/bootstrap4.rst

@@ -12,10 +12,10 @@ template which other templates extend from):
 
     {# beware that the blocks in your template may be named different #}
     {% block head_css %}
-        <!-- Copy CSS from https://getbootstrap.com/docs/4.0/getting-started/introduction/#css -->
+        <!-- Copy CSS from https://getbootstrap.com/docs/4.1/getting-started/introduction/#css -->
     {% endblock %}
     {% block head_js %}
-        <!-- Copy JavaScript from https://getbootstrap.com/docs/4.0/getting-started/introduction/#js -->
+        <!-- Copy JavaScript from https://getbootstrap.com/docs/4.1/getting-started/introduction/#js -->
     {% endblock %}
 
 If your application uses modern front-end practices, it's better to use
@@ -111,6 +111,6 @@ Form errors are rendered **inside** the ``<label>`` element to make sure there
 is a strong connection between the error and its ``<input>``, as required by the
 `WCAG 2.0 standard`_.
 
-.. _`their documentation`: https://getbootstrap.com/docs/4.0/
+.. _`their documentation`: https://getbootstrap.com/docs/4.1/
 .. _`WCAG 2.0 standard`: https://www.w3.org/TR/WCAG20/
-.. _`custom forms`: https://getbootstrap.com/docs/4.0/components/forms/#custom-forms
+.. _`custom forms`: https://getbootstrap.com/docs/4.1/components/forms/#custom-forms

form/form_customization.rst

@@ -857,6 +857,6 @@ more details about this concept in Twig, see :ref:`twig-reference-form-variables
 .. _`bootstrap_4_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig
 .. _`bootstrap_4_horizontal_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_horizontal_layout.html.twig
 .. _`Bootstrap 3 CSS framework`: https://getbootstrap.com/docs/3.3/
-.. _`Bootstrap 4 CSS framework`: https://getbootstrap.com/docs/4.0/
+.. _`Bootstrap 4 CSS framework`: https://getbootstrap.com/docs/4.1/
 .. _`foundation_5_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/foundation_5_layout.html.twig
 .. _`Foundation CSS framework`: http://foundation.zurb.com/