Update form_builder.rst

Update import statement to include all classes used in the example

docs/core_components/form_builder.rst

@@ -23,7 +23,11 @@ Within the models.py of one of your apps, create a model that extends wagtailfor
 
 
 .. code:: python
-
+    
+    from modelcluster.fields import ParentalKey
+    from wagtail.wagtailadmin.edit_handlers import (FieldPanel, InlinePanel,
+        MultiFieldPanel)
+    from wagtail.wagtailcore.fields import RichTextField
     from wagtail.wagtailforms.models import AbstractEmailForm, AbstractFormField
 
     class FormField(AbstractFormField):