Broken link for Diagnosticable API in README

[royarg02]

Describe the bug There is a broken link in README directing to the Diagnosticable API. https://github.com/rrousselGit/provider/blob/4bea63e1f30df2aba65e277ae54ac087cc0e8bfc/README.md?plain=1#L354

Diagnosticable was converted to a mixin, the correct link should be: https://api.flutter.dev/flutter/foundation/Diagnosticable-mixin.html