docs: Correct typos in --help and CONTRIBUTING.md (#2695)

## Overview

Fix typos in `--help` output and CONTRIBUTING.md.

## Testing

None.

## Checklist

- [X] I have signed the [Contributor License
Agreement](https://cla.developers.google.com/).
- [ ] I have run the linter using `./scripts/run_lints.sh`.
- [ ] I have run the unit tests using `./scripts/run_tests.sh`.
- [X] I have made my commits and PR title follow the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) specification.

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Co-authored-by: Xueqin Cui <72771658+cuixq@users.noreply.github.com>