[Minor][Doc] Fix issues in the documentation

[lixueclaire]

Proposed changes

as titled.

Checklist

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• [x] I have read the CONTRIBUTING doc
• [x] I have signed the CLA
• [ ] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)