Update the error message accordingly

Co-authored-by: Matt Smith <[email protected]>

Author: seratch

slackeventsapi/server.py

@@ -21,7 +21,7 @@ def __init__(self, signing_secret, endpoint, emitter, server):
             if isinstance(server, Flask) or isinstance(server, Blueprint):
                 self.bind_route(server)
             else:
-                raise TypeError("Server must be an instance of Flask")
+                raise TypeError("Server must be an instance of Flask or Blueprint")
         else:
             Flask.__init__(self, __name__)
             self.bind_route(self)