Fix broken reference to forms/bootstrap4

Fix typo in reference.

Fixes the following diagnostic emitted by `_build/build.php`:
```
Found invalid reference "reference-forms-bootstrap4-error-messages" in file "form/form_customization"
```

Author: MarkPedron

form/bootstrap4.rst

@@ -77,7 +77,7 @@ If you prefer to apply the Bootstrap styles on a form to form basis, include the
         {{ form(form) }}
     {% endblock %}
 
-.. _reference-forms-bootstrap-error-messages:
+.. _reference-forms-bootstrap4-error-messages:
 
 Error Messages
 --------------