[Doc] fix quickstart (#548)

Signed-off-by: David Chen <530634352@qq.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/getting_started/quickstart.md

@@ -35,7 +35,8 @@ from vllm_omni.entrypoints.omni import Omni
 if __name__ == "__main__":
     omni = Omni(model="Tongyi-MAI/Z-Image-Turbo")
     prompt = "a cup of coffee on the table"
-    images = omni.generate(prompt)
+    outputs = omni.generate(prompt)
+    images = outputs[0].request_output[0]["images"]
     images[0].save("coffee.png")
 ```