Follow PEP8 convention in documentation

This amends the documentation for the multi-range slider to include
spaces after commas, following PEP8.

Author: pbugnion

docs/source/examples/Widget List.ipynb

@@ -639,11 +639,11 @@
    "outputs": [],
    "source": [
     "import datetime\n",
-    "dates = [datetime.date(2015,i,1) for i in range(1,13)]\n",
+    "dates = [datetime.date(2015, i, 1) for i in range(1, 13)]\n",
     "options = [(i.strftime('%b'), i) for i in dates]\n",
     "widgets.SelectionRangeSlider(\n",
     "    options=options,\n",
-    "    index=(0,11),\n",
+    "    index=(0, 11),\n",
     "    description='Months (2015)',\n",
     "    disabled=False\n",
     ")"