[MISC] Fix invalid escape sequence '\' (#8830)

Signed-off-by: Peter Pan <Peter.Pan@daocloud.io>

benchmarks/benchmark_serving.py

@@ -1,4 +1,4 @@
-"""Benchmark online serving throughput.
+r"""Benchmark online serving throughput.
 
 On the server side, run one of the following commands:
     vLLM OpenAI API server
@@ -963,4 +963,4 @@ if __name__ == "__main__":
     )
 
     args = parser.parse_args()
-    main(args)
+    main(args)