Fix package name in FastAPI README (OpenAPITools#20371)

Author: DavidRV00

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

@@ -19,7 +19,7 @@ To run the server, please execute the following from the root directory:
 
 ```bash
 pip3 install -r requirements.txt
-PYTHONPATH=src uvicorn openapi_server.main:app --host 0.0.0.0 --port {{serverPort}}
+PYTHONPATH=src uvicorn {{packageName}}.main:app --host 0.0.0.0 --port {{serverPort}}
 ```
 
 and open your browser at `http://localhost:{{serverPort}}/docs/` to see the docs.