Fixed documentation - RasaHQ#8069

Co-autohored-by: rafz <rafzteodosio@gmail.com>
Co-authored-by: marcelo <santos46ma@gmail.com>

docs/docs/forms.mdx

@@ -510,7 +510,7 @@ Again, it is **strongly** recommended that you use
 As soon as the form determines which slot has to be filled next by the user, it will
 execute the action `utter_ask_<form_name>_<slot_name>` or `utter_ask_<slot_name>`
 to ask the user to provide the necessary information. If a regular utterance is not
-enough, you can also use a custom action `action_ask_<form_name>__<slot_name>` or
+enough, you can also use a custom action `action_ask_<form_name>_<slot_name>` or
 `action_ask_<slot_name>` to ask for the next slot.
 
 ```python