Optimize the `benchmark` docs

[dl239]

https://raw.githubusercontent.com/4paradigm/OpenMLDB/main/benchmark/README.md

• update step2 as below

2. copy the configuration and package
    ```bash
    mkdir -p /work/benchmark/conf /work/benchmark/lib
    cp target/openmldb-benchmark-0.5.0.jar  /work/benchmark/lib
    cp src/main/resources/conf.properties /work/benchmark/conf
    ```

• update command in step4 as below java -cp conf/:lib/* com._4paradigm.openmldb.benchmark.OpenMLDBPerfBenchmark