give hint on where to find custom.js

examples/notebooks/User Interface.ipynb

@@ -375,7 +375,7 b''
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "If you want your keyboard shortcuts to be active for all of your notebooks, put the above API calls into your `custom.js` file."
+      "If you want your keyboard shortcuts to be active for all of your notebooks, put the above API calls into your `<profile>/static/custom/custom.js` file."
      ]
     },
     {