Fixing virtual_packet_logging instructions for line info widget (#1575)

docs/using/visualization/generating_widgets.ipynb

@@ -574,6 +574,17 @@
     "![Line Info Widget Demo](../images/line_info_widget_demo.gif)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "<div class=\"alert alert-info\">\n",
+    "\n",
+    "**Important:** The virtual packet logging capability must be active in order to produce virtual packets' spectrum in `Line Info Widget`. Thus, make sure to set `virtual_packet_logging: True` in your configuration file. It should be added under `virtual` property of `spectrum` property, as described in [configuration schema](https://tardis-sn.github.io/tardis/using/components/configuration/configuration.html#spectrum).\n",
+    "\n",
+    "</div>"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -598,7 +609,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.9"
+   "version": "3.8.8"
   },
   "notify_time": "5",
   "toc": {