fix the helloworld example

The helloworld example did not run as the port (9090) was passed as the
server parameter.

examples/helloworld/hello.py

@@ -7,6 +7,6 @@ def post_greeting(name: str) -> str:
     return 'Hello {name}'.format(name=name)
 
 if __name__ == '__main__':
-    app = connexion.FlaskApp(__name__, 9090, specification_dir='swagger/')
+    app = connexion.FlaskApp(__name__, port=9090, specification_dir='swagger/')
     app.add_api('helloworld-api.yaml', arguments={'title': 'Hello World Example'})
     app.run()