Bump async from 2.6.3 to 2.6.4 in /docs-src

Bumps [async](https://github.com/caolan/async) from 2.6.3 to 2.6.4.
- [Release notes](https://github.com/caolan/async/releases)
- [Changelog](https://github.com/caolan/async/blob/v2.6.4/CHANGELOG.md)
- [Commits](caolan/async@v2.6.3...v2.6.4)

---
updated-dependencies:
- dependency-name: async
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>