fix wrong link to endpoint

Community member pointed out that link to the endpoint does not work. It is because of the `.html` after `http-api`. The current link works when you run `make livehtml`; however, it does not work when the docs are deployed on `rasa.com/docs`. Not sure how to make sure my change works

Author: TyDunn

docs/core/actions.rst

@@ -184,7 +184,7 @@ for example to display the output of a long running background operation
 or notify the user of an external event.
 
 To do so, you can ``POST`` to this
-`endpoint <../../api/http-api.html/#operation/executeConversationAction>`_ ,
+`endpoint <../../api/http-api/#operation/executeConversationAction>`_ ,
 specifying the action which should be run for a specific user in the request body. Use the
 ``output_channel`` query parameter to specify which output
 channel should be used to communicate the assistant's responses back to the user.