Adding 'widgets.' prefix to doc to fix compile error.

Author: adam-ah

docs/source/examples/Widget List.ipynb

@@ -557,9 +557,9 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "Box(\n",
+    "widgets.Box(\n",
     "    [\n",
-    "        Label(value='Pizza topping with a very long label:'), \n",
+    "        widgets.Label(value='Pizza topping with a very long label:'), \n",
     "        widgets.RadioButtons(options=['pepperoni', 'pineapple', 'anchovies', 'and the long name that will fit fine'])\n",
     "    ]\n",
     ")"