Fix grammar docs for type aliases in AST

Closes https://github.com/dart-lang/sdk/pull/52515

GitOrigin-RevId: aa7b3ddee2a5b90bbb671e15b9a777ae441f83b4
Change-Id: Ib201d5ef9193b50305138efc23e97b8c5be8336f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/305840
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

https://dart.googlesource.com/sdk/+/e56ef707eb6eb0480de30a37715bc0bb50978bb3