[DOCFIX] Update StressBench.md
[DOCFIX] Update StressBench.md

### What changes are proposed in this pull request?

Update stressbench doc

### Why are the changes needed?

mark it experimental

### Does this PR introduce any user facing changes?

na

pr-link: Alluxio/alluxio#14097
change-id: cid-997ffabbd0f93486f03a187d3e25e71e355549c2
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