Wrong example signature (#3686)

Callback should follow the `_{node-id}_{event}`, but example uses a double underscore (which is not working)

Co-authored-by: Philipp Rudiger <prudiger@anaconda.com>

examples/user_guide/Custom_Components.ipynb

@@ -178,7 +178,7 @@
     "    _dom_events = {'input': ['change']}\n",
     "```\n",
     "\n",
-    "Once subscribed the class may also define a method following the `_{node-id}_{event}` naming convention which will fire when the DOM event triggers, e.g. we could define a `__custom_id_change` method. Any such callback will be given a `DOMEvent` object as the first and only argument.\n",
+    "Once subscribed the class may also define a method following the `_{node-id}_{event}` naming convention which will fire when the DOM event triggers, e.g. we could define a `_custom_id_change` method. Any such callback will be given a `DOMEvent` object as the first and only argument.\n",
     "\n",
     "The `DOMEvent` contains information about the event on the `.data` attribute, like declaring the type of event on `.data.type`."
    ]