Made the name of environment variables consistent through benchmarking doc

[sauraank]

Description of change

In our Benchmarking doc, the names for environment variables S3_BUCKET_NAME, S3_BUCKET_TEST_PREFIX, S3_BUCKET_BENCH_FILE and S3_BUCKET_SMALL_BENCH_FILE across steps. This might create confusion for new user trying to run benchmark.

Made the names consistent across the steps for environment variables in used in benchmarking.

Relevant issues: NA

Does this change impact existing behavior?

No. Just documentation edits.

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