[DOCFIX] Update readme.md for tracing
[DOCFIX] Update readme.md for tracing

### What changes are proposed in this pull request?
Update readme for tracing

### Why are the changes needed?
Fix formatting

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

No

pr-link: Alluxio/alluxio#13661
change-id: cid-7b31f34bf696b0b88e69f89363ca6046d0cb85fd
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