🐛 fix name of shutdown_event in docs (#105)

Fix name copy/paste name error in docs source for startup/shutdown events.

docs/src/events/tutorial002.py

@@ -4,7 +4,7 @@ app = FastAPI()
 
 
 @app.on_event("shutdown")
-def startup_event():
+def shutdown_event():
     with open("log.txt", mode="a") as log:
         log.write("Application shutdown")