[Doc] go through the nydus repo doc and fix issues

[imeoer]

[huiboxes]

Hello, I noticed a few typos and formatting issues in the docs and would like to fix them:

1. chunk-deduplication.md: "Probntroduction" → "Introduction"
2. nydus-design.md: "Raft" → "Rafs", "meterials" → "materials"
3. nydus-fscache.md: step numbering (5-9 → 4-8)

I'll submit a PR for your review. Please let me know if you have any feedback. Thanks.