build: bump multipart-post from 2.1.0 to 2.1.1 in /docs

Bumps [multipart-post](https://github.com/nicksieger/multipart-post) from 2.1.0 to 2.1.1.
- [Release notes](https://github.com/nicksieger/multipart-post/releases)
- [Changelog](https://github.com/socketry/multipart-post/blob/master/History.txt)
- [Commits](https://github.com/nicksieger/multipart-post/compare/v2.1.0...v2.1.1

)

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