Fix incorrect import

Author: mwcraig

notebooks/06.Layout/06.03-more-flexbox-layout.ipynb

@@ -327,7 +327,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "from ipywidgets import Layout, Button, Box, Label\n",
+    "from ipywidgets import Layout, Button, VBox, Label\n",
     "\n",
     "item_layout = Layout(height='100px', min_width='40px')\n",
     "items = [Button(layout=item_layout, description=str(i), button_style='warning') for i in range(40)]\n",