Update `String.replaceFirstMapped` documentation.
Update `String.replaceFirstMapped` documentation.

The result of `replace` must be a String, so no need to mention any other type in the documentation.

Fixes #30248.
BUG= http://dartbug.com/30248
R=lrn@google.com

Review-Url: https://codereview.chromium.org/2994053003 .

dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk