Bump markdownlint from 0.21.0 to 0.21.1 (#122)

Bumps [markdownlint](https://github.com/DavidAnson/markdownlint) from 0.21.0 to 0.21.1.
- [Release notes](https://github.com/DavidAnson/markdownlint/releases)
- [Commits](DavidAnson/markdownlint@v0.21.0...v0.21.1)

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