Add import for required

flask_admin/model/base.py

@@ -311,7 +311,8 @@ class BaseModelView(BaseView, ActionsMixin):
         list of possible options.
 
         Example::
-
+        
+            from wtforms.validators import required
             class MyModelView(BaseModelView):
                 form_args = dict(
                     name=dict(label='First Name', validators=[required()])