[Doc] fix quickstart (#620)

Signed-off-by: David Chen <530634352@qq.com>

docs/getting_started/quickstart.md

@@ -36,7 +36,7 @@ if __name__ == "__main__":
     omni = Omni(model="Tongyi-MAI/Z-Image-Turbo")
     prompt = "a cup of coffee on the table"
     outputs = omni.generate(prompt)
-    images = outputs[0].request_output[0]["images"]
+    images = outputs[0].request_output[0].images
     images[0].save("coffee.png")
 ```