Run the server using PYTHONPATH and correct module (OpenAPITools#17820)

Executing the command from the README file should allow the user to run the application. Without the proposed changes either the error message `Error loading ASGI app. Could not import module "main".` or `ModuleNotFoundError: No module named 'openapi_server'` is being returned.

Author: AlexanderWillner

modules/openapi-generator/src/main/resources/python-fastapi/README.mustache

@@ -18,7 +18,7 @@ To run the server, please execute the following from the root directory:
 
 ```bash
 pip3 install -r requirements.txt
-uvicorn main:app --host 0.0.0.0 --port {{serverPort}}
+PYTHONPATH=src uvicorn openapi_server.main:app --host 0.0.0.0 --port {{serverPort}}
 ```
 
 and open your browser at `http://localhost:{{serverPort}}/docs/` to see the docs.