Broken hyperlink in dart doc for ControlValueAccessor class

[benasocj]

If you search for the hyperlink named "ControlValueAccessor" on https://angular.io/docs/dart/latest/guide/template-syntax.html and click on it, it leads to a 404.

[chalin]

Thanks for reporting this. The issue is broader in scope and will be fixed as part of #2173, so we'll close this issue and track progress in #2173.