minor symfony#15982 Removing duplicate "Bootstrap 4 form theme" (ThomasLandauer)

This PR was merged into the 4.4 branch.

Discussion
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Removing duplicate "Bootstrap 4 form theme"

Commits
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54622e6 Removing duplicate "Bootstrap 4 form theme"

Author: javiereguiluz

form/form_customization.rst

@@ -257,8 +257,8 @@ Renders any errors for the given field.
 
 .. caution::
 
-    In the Bootstrap 4 form theme, ``form_errors()`` is already included
-    in ``form_label()``, see ":ref:`reference-forms-bootstrap-error-messages`"
+    In the :ref:`reference-forms-bootstrap4-error-messages`, ``form_errors()``
+    is already included in ``form_label()``.
 
 .. _reference-forms-twig-widget: