[Flutter] Remove invalid link in "Built-in Types" section

[ksatoudeveloper]

Roadmap URL

https://roadmap.sh/flutter

Suggestions

The link in the "Built-in Types" section is invalid:
https://dart.dev/guides/language/coming-from/js-to-dart#built-in-types

The content was removed via this PR:
dart-lang/site-www#7139

Please remove this link.

[jcanalesluna]

Thanks for reporting. I've just fixed it!