Update docstring of connexion.ConnexionMiddleware.add_api (#2030)

Changes proposed in this pull request:
- Update docstring of `connexion.ConnexionMiddleware.add_api` to match
return value to actual return.

---------

Co-authored-by: Ruwann <[email protected]>

Author: bastantoine

connexion/middleware/main.py

@@ -402,8 +402,6 @@ def add_api(
         :param kwargs: Additional keyword arguments to pass to the `add_api` method of the managed
             middlewares. This can be used to pass arguments to middlewares added beyond the default
             ones.
-
-        :return: The Api registered on the wrapped application.
         """
         if self.middleware_stack is not None:
             raise RuntimeError("Cannot add api after an application has started")