Replace missed warning_once for rerank API (#12472)

Signed-off-by: mgoin <michael@neuralmagic.com>

vllm/entrypoints/openai/api_server.py

@@ -528,7 +528,7 @@ async def do_rerank(request: RerankRequest, raw_request: Request):
 @router.post("/v1/rerank")
 @with_cancellation
 async def do_rerank_v1(request: RerankRequest, raw_request: Request):
-    logger.warning(
+    logger.warning_once(
         "To indicate that the rerank API is not part of the standard OpenAI"
         " API, we have located it at `/rerank`. Please update your client"
         "accordingly. (Note: Conforms to JinaAI rerank API)")