[Bugfix] Align Hub kernel dependency guidance

Signed-off-by: hsliuustc0106 <liuhongsheng4@huawei.com>

docs/user_guide/diffusion/attention_backends/huggingface_hub.md

@@ -12,7 +12,7 @@ implementations to reduce numerical drift and sampling divergence.
 ## Installation
 
 ```bash
-pip install kernels==0.16.0
+pip install kernels==0.15.2
 ```
 
 If `kernels` is unavailable, an explicit `FLASH_ATTN_HUB` or