[Bug][CI] Pin the LTX FP8 quality gate to its established sigma schedule (#5302)

Signed-off-by: mglyn <1203789601@qq.com>

examples/online_serving/text_to_video/README.md

@@ -254,7 +254,7 @@ done
 
 ```bash
 vllm serve Lightricks/LTX-2 --omni --port 8098 \
-    --enforce-eager --flow-shift 1.0 --boundary-ratio 1.0
+    --enforce-eager --boundary-ratio 1.0
 ```
 
 #### Start with Optimization Presets