[Form] Minor deletion

Follow-up of #15982
The point is that it's included in **label** (not in *errors*) ;-)

Author: ThomasLandauer

form/form_customization.rst

@@ -257,7 +257,7 @@ Renders any errors for the given field.
 
 .. caution::
 
-    In the :ref:`error messages of Bootstrap 4 Form Theme <reference-forms-bootstrap4-error-messages>`,
+    In the :ref:`Bootstrap 4 Form Theme <reference-forms-bootstrap4-error-messages>`,
     ``form_errors()`` is already included in ``form_label()``.
 
 .. _reference-forms-twig-widget: