docs(changelog): add missing link

Author: dignifiedquire

CHANGELOG.md

@@ -698,6 +698,7 @@ task::blocking(async {
 - Initial beta release
 
 [Unreleased]: https://github.com/async-rs/async-std/compare/v1.5.0...HEAD
+[1.6.0-beta.1]: https://github.com/async-rs/async-std/compare/v1.4.0...v1.6.0-beta.1
 [1.5.0]: https://github.com/async-rs/async-std/compare/v1.4.0...v1.5.0
 [1.4.0]: https://github.com/async-rs/async-std/compare/v1.3.0...v1.4.0
 [1.3.0]: https://github.com/async-rs/async-std/compare/v1.2.0...v1.3.0