update benchmark README to use --fp8-gemm-backend instead of env var#15689

Fridge003 merged 1 commit intosgl-project:mainsgl-project/sglang:mainfrom

leejnau:dsv3benchmark_remove_gemm_envvar_use_flagleejnau/sglang:dsv3benchmark_remove_gemm_envvar_use_flagCopy head branch name to clipboard