`Runes` docs reference nonexistent part of site

[treeplate]

The main docs for Runes (https://api.dart.dev/dart-core/Runes-class.html or https://main-api.flutter.dev/flutter/dart-core/Runes-class.html) mention in the "See also" section https://dart.dev/language#runes-and-grapheme-clusters - that link doesn't seem to work right.

[FMorschel]

CC @parlough