Add missing anchor in forms/bootstrap5

Fixes the following diagnostic emitted by `_build/build.php`:
```
Found invalid reference "reference-forms-bootstrap5-error-messages" in file "form/form_customization"
```

Author: MarkPedron

form/bootstrap5.rst

@@ -83,6 +83,8 @@ If you prefer to apply the Bootstrap styles on a form to form basis, include the
     container. If you override the ``row_attr`` class option, the ``mb-3`` will
     be overridden too and you will need to explicitly add it.
 
+.. _reference-forms-bootstrap5-error-messages:
+
 Error Messages
 --------------