Bump grunt-markdownlint from 3.1.4 to 3.1.5

Bumps [grunt-markdownlint](https://github.com/sagiegurari/grunt-markdownlint) from 3.1.4 to 3.1.5.
- [Release notes](https://github.com/sagiegurari/grunt-markdownlint/releases)
- [Changelog](https://github.com/sagiegurari/grunt-markdownlint/blob/master/docs/CHANGELOG.md)
- [Commits](https://github.com/sagiegurari/grunt-markdownlint/commits/v3.1.5)

---
updated-dependencies:
- dependency-name: grunt-markdownlint
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>