Update Benchmark README.md to reflect actual options in runner.py

[npankaj365]

Updates example command lines in README.md to use no_istio instead of baseline to match runner.py's implemented arguments

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[linux-foundation-easycla]

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[istio-testing]

Hi @npankaj365. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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