Fix widget dropdown example

Thanks to @parente in  #7350 for spotting this.

examples/Interactive Widgets/Widget List.ipynb

@@ -298,8 +298,8 b''
    "source": [
     "from IPython.display import display\n",
     "w = widgets.Dropdown(\n",
-    "    values=[1, 2, 3],\n",
-    "    value=2,\n",
+    "    values=['1', '2', '3'],\n",
+    "    value='2',\n",
     "    description='Number:',\n",
     ")\n",
     "display(w)"
@@ -565,16 +565,22 b''
  ],
  "metadata": {
   "kernelspec": {
-   "codemirror_mode": {
-    "name": "python",
-    "version": 2
-   },
    "display_name": "Python 2",
-   "language": "python",
    "name": "python2"
   },
-  "signature": "sha256:83b39d018a7a6ae0a324b9f3d38debafbfb2ed0a114e4bbd357fb318f8f23438"
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.8"
+  }
  },
  "nbformat": 4,
  "nbformat_minor": 0
-}
  No newline at end of file
+}