[Doc] Fix Qwen3-Omni modality documentation examples (#5173)

Signed-off-by: psv666 <2693925048@qq.com>
Co-authored-by: amy-why-3459 <wuhaiyan17@huawei.com>

examples/online_serving/qwen3_omni/README.md

@@ -353,13 +353,16 @@ curl http://localhost:8091/v1/chat/completions \
 #### Text + Audio
 
 ```bash
-curl -s http://localhost:8091/v1/chat/completions \
+response=$(curl -s http://localhost:8091/v1/chat/completions \
   -H "Content-Type: application/json" \
   -d '{
     "model": "Qwen/Qwen3-Omni-30B-A3B-Instruct",
     "messages": [{"role": "user", "content": "Describe vLLM in brief."}],
-    "modalities": ["audio"]
-  }' | jq -r '.choices[0].message.audio.data' | base64 -d > output.wav
+    "modalities": ["text", "audio"]
+  }')
+
+echo "$response" | jq -r '.choices[0].message.content'
+echo "$response" | jq -r '.choices[1].message.audio.data' | base64 -d > output.wav
 ```
 
 ### Using Python client
@@ -456,8 +459,8 @@ response = client.chat.completions.create(
     modalities=["audio"],
     extra_body={"speaker": "chelsie"}
 )
-# Audio uses the specified speaker
-print(response.choices[1].message.audio)
+# Audio-only responses contain a single audio choice
+print(response.choices[0].message.audio)
 ```
 
 Supported speaker names depend on the model (e.g. `Ethan`, `Chelsie`, `Aiden`). Omit `speaker` to use the default.